I already started this at 16, my "customers" have been only local (idk how much I should trust shipping a PC, they're fragile, but I shipped two cross country so far and it worked okay)
I used two PC stores as my motivation : Alienware, which is easy to out-do because I noted that each of their PC's is at least $500 more than it is worth (usually way more) and the systems have crap for cooling. I wanted to build the high-end seekers a machine they'll enjoy at a decent ish price (locally, again) And I used best buy as another. I noted some office type desktops had an i7 6700 (K) on them! Which I found crazy. I made some parts list and originally started with some flyers near this "gaming hub" downtown (it's basically a nerd lounge, haha) and I got a few who were interested. It was simple enough for me...
As for building fees, I went with $30. I used similar logic to you, in the brochure I left on the bulletin board at said lounge. Basically "The Alienware you're eyeing- Runs too hot, awfully expensive, and likely has software on it already, slowing you down. I can build you something better."
I made an alternate email and left that in the brochure as a contact to iron out details. For instance, one person contacted me and said "I need something that blows away my AMD rig right now (9590, HD 7870 x2) I was looking at an Area-51 from Alienware and it looked promising. Can you really build me something better, cheaper?
I asked him which one, he was looking at a 6800k + GTX 970.
I then went through the process ; You know that in most games, you won't need 6 cores? The i7 6700k has fewer cores, but they're stronger so they'll likely help you.
You mentioned 4k a little while ago. I think you'll like having an NVIDIA GTX 1080 with you if you ever want to make the jump to that.
Do you really need 4 TB HDD Space?
Etc. Etc.
My simple procedure said that parts and labor are bought after funds are exchanged, so I used paypal (cause it was simple) and he paid for the rig + build fee, etc. and I ordered it and got to work. About 2 days later I drove it down to the lounge, and later that night I got the email (wow this is so amazing)
Here's a few things to consider
-As brought up before, why pay you $50 to build? At least make your reasoning very clear. It's evident to me that build > buy because I love building and all my builds have worked great for me.
-Why wait for a long time to ship parts, wait for you to build, THEN you either ship it or bring it down yourself when I can get one right away out of best buy?
(kinda same reasoning + patience)
-Make sure you have a clear payment method. I used paypal for these people at the lounge because I noted that younger people tended to favor it. I built a couple home-office desktops for older people who just gave me a check. Choose one and do a little research on how it works if you don't already know.
-Warranty? They probably won't know what the parts are, so you either need to make it clear "I do not fix your PC after it becomes yours" etc. or something clearly. I simply say "if something happens in the first 3 months then call me and I'll try my best to get it proper (usually you can return faulty parts, be careful who you choose and make sure you can do that if it's the case)
-If you ship across the country, shipping a large PC and insuring that package is expensive! That's mostly why I decided to keep it local except for a few special cases.
I encourage you to dive deeper into this, I find doing it more fun than earning minimum wage cashiering for grumpy customers at a supermarket or something. But do know what you're getting into.