You know someone at a lumber company?
Afraid I put some sort of markup on the lumber?
Have some other reason you think it might be better if you get the lumber?

Ok. But here are some things to consider:

Will you be taking the plan I draw and making a material list with it? Or do I still need to do that? And do you know if your lumber company will carry everything we need, and if not are you prepared to also deal with multiple lumber companies? Are you prepared to go there or were you just going to make a call? Because it's not really that simple.

Even if I make the list and give it to you, there is still a lot to deal with when it comes to the lumber.
When you or your friend (son/father/uncle) get the lumber it adds another middle man in between me and the project I'm trying to run.

If I need more lumber, I have to tell you (and then you may be mad that it's costing more).
If I need to exchange some of it because they sent some bad boards, it makes it more difficult. You either have to do it or I have to work through you to do it. If you don't have an account then they try to give me your refund. Oh wait, no they don't because I don't even have your receipt so I can't even try to exchange or take back your boards.

If you set it up to be delivered and it's not showing up, I have to call you and ask who you talked to and who told you when it was coming. That's when we find out that you didn't know which guy is known for dropping the ball at that lumber company - or that it was your friend who IS that guy but you didn't know that like I do. If some wrong boards show up, will that be because my list was wrong (not likely), because you read it wrong, or because your lumber guy read it wrong. You see, the people I deal with know me (and I know they are the good ones) and even if something is wrong on the list they will catch it - they know which size concrete bags we want and that all the boards are treated (and they know the few exceptions to those rules).

When the job is done and we have some scraps that we could use for blocking and other things on the next job, will I just be able to take those like I always do or will you want to charge me for those if I take them? I brought some things to your house from the scraps off the last job, so it works out if I keep your scraps for my next job. But you won't want to keep them or give them away, so.... It's another issue we have to discuss.

Plus, if you're getting any sort of wood handrail, I always go to Lowe's to hand-pick each and every board for that since they are the only ones that have enough good looking railing boards to choose from. If the guys at any lumber company pick our stuff out for us, to deliver or have us pick up, they will pick out crappy boards that you would NOT want to see on your railing. So are you going to go pick out and load up all the handrail boards? Do you know what you're looking for? Do you know what "cup" and "crown" is? Do you know which cuts at the mill will produce boards you don't want on your rail?

It's just better if we stick to the thing where I'm selling you a completed deck and I deal with all the headaches in between. I don't put a markup on the lumber, but there is LOT I do concerning the lumber that I promise you won't want to deal with. 
In my younger days I mistakenly let a client get the lumber. And I was in hell the whole time. Every time I needed to make a move concerning the lumber I had to ask them first, which meant I had to wait on them to get back with me and then wait while they contacted their lumber guy and then wait while they explained it the wrong way to their lumber guy and then wait again for them to have time to finally get back to me to tell me when it would be resolved and we could finally make forward progress again - all the while guys are standing around on MY clock. I get in a bad mood when I lose the very little money I'm making over something that could have been avoided.

If I were to ever agree to let a client get the lumber again, my labor would be quite a bit higher for the headaches I know I'll be dealing with. So therefore I won't be doing any project where it's not Turn Key; where I'm not the one dealing with every aspect myself and managing all the money for the project.

No, part of it isn't that I just don't want you to know how much I may be making on the job. Because for you to know that you'd also have to know how much my General Liability and Worker Comp insurances are, how much my bonds are, how much I pay my guys, how much the tools and other consumables cost, and more. The money after the lumber is paid is not all Profit. So it's not that.

It's that we can't have two cooks in the kitchen. Only one of us needs to be managing the project and the money for the project. Trust me, you won't like it either if you get the lumber - you'd be paying more for the whole thing and you'd be working your job and part of mine during the project. Even if you think you'll get a discount on the lumber because you know a guy, that discount and more will be eaten up by all the problems it will cause.

And it's really pretty telling when a person wants to get their own lumber. It tells me they don't trust me right off the bat; that they are afraid they'll be paying too much if I handle everything. And I don't want to be in any relationship where the other person doesn't trust me before they even meet me.

Furthermore, we don't have enough lumber companies around here for the demand as it is, so they are all making plenty of money and no one will go hungry if we don't get your deck package from the guy you know. Plus, if he gives you a discount, won't that eat up the commission he would get by you buying from him? So it's not really helping him. Or will he raise the cost to cover the discount? (You do know that is the most used sales tactic in all of retail, don't you? Raise the price and then give a discount.) Hmmmm.... now who should we not trust? It might be best for your friendship if you don't get the lumber from him!

Anyway, the only way I work is where you pay me to manage the entire job; you pay me a set price and I deal with everything so you don't have to... all the problems that DO come up... and you get a finished deck in the end and I get the scraps AND the trash.

I do not work any other way.

I've been doing this for over 30 years and I've got it down to a science. I know what problems can come up, I do things to avoid most of them, and I can roll with the ones that can't be avoided because I don't have to ask anyone permission just to start handling issues. If you insert yourself between me and what I do best then that makes it much harder on me (and you) and I will be making sure I'm compensated for having to do things the hard way.

So if you insist on getting the lumber, please let me know up front so we can save each other some time and you can find a contractor that will work that way and I can move on to the next client that has faith that I'm working in their best interest.. 

But if you're cool with me doing what I do and keeping your headaches and costs as low as possible, and you paying a fixed price so I am the one responsible for everything, then I can't wait to work with you!