You might have a harder time finding a GC to work w/you if you supply the goods. Yea, there will be some markup on the goods but, I'd negotiate the total package to offset the markup. Your gonna have to pay somewhere, these guys gotta eat too. At the least, this gives them an out as far as a warranty and/or workmanship would go. Can you hear it now?: "Hey, we just installed what you supplied, if it were me I would have went w/...."