he should buy the parts. Pay you for your services to assemble the computer. You buy parts then you are responsible for DOA, Return shipping costs, Trouble shooting, Foot the bill on damaged parts non returnable.
IF you buy the parts you mark everything up by at least 15-20% per part, mark up shipping, and add assembly fee. You are taking all the responsibility and risk while he just throws a few hundred at you for your efforts.
Make him buy his parts based on what you are able to build for him. Make him responsible for returns, costs for returns, shipping, DOA parts and all the problems with dealing with RMA