20 years ago we built our house, and took on most of the GC responsibilities (in order to meet budget); the general / bidder's crew did some of the work, we hired many subcontractors and suppliers that he had good quotes from, etc. Yes, we saved his markup, but we earned every dime. If you have the time to source odds and ends, the ability to be there to receive deliveries and secure them against theft, exchange defective items, and if you can think your way through the whole project to cover the costs and installation of each piece, go for it. All of the handling and arranging and ordering does take time and costs someone.
And, there are cash flow issues which come up -- some suppliers require payment when you place the order, and the lender will pay you only after the item is installed. (Cabinet shops may require payment for the cabinets when you order, and then bill you for the installation when complete, but the bank wants to see/will pay for only those items that are physically attached, are thus part of the real estate, and thereby secured by your loan.) If the contractor is responsible, (and if your contract with them says so), you can point out to them that the lender hasn't paid you yet, but you will pay as soon as you can. If you already have your construction lender selected, attach their payment schedule to the construction contract - this will give you some relief.
Strive to be fair with the contractor. Remember that how this project goes for them is also a marketing experience for you -- they may recommend you to others, not only because of the quality of your design, but because you treated them decently. Good relations during construction will make any call-back work easier.
And, be aware that there are several levels of pricing. Retail - you shouldn't have to pay this very often; "Contractor", "wholesale", "trade", etc. You may need to establish a contractor's account at one or more local building supply outlets; ideally the sales tax will be taken care of there .... The whopping 20% "architect's" discount you might be offered, off of retail, might pale in comparison to the 40% or so that the electrician gets for the same light fixture, just because the electrician is a repeat customer and you are not. We got a great price on most of our lights by ordering directly from the factory, but I spent an afternoon at the freight company unloading the monster shipping box, since their trucks were too big for our driveway. Yes, we came out ahead, but it would have been nice to have been able to have logged that time on my timecard.
-------------------------------------------
Joel Niemi AIA
Snohomish WA
-------------------------------------------