I understand both sides of this issue and to be honest there is not really a good solution.

The installer/retailer will always feel a little resentful by some PITA customer telling him how much he can get something somewhere else for. I try to do my own research without bothering the pros. I then use the net to get my best price. If it is something I can't find or can't do myself I call a pro get his price and either accept it or reject it but I don't ask him to lower it. I just don't think that is right. He obviously gave me that price because that is what he feels the job is worth to him. I may get a couple bids but I never ask for a lower fee. If you ask him to lower it then he may have to leave something out to still turn a profit. I would rather have it done right without skimping. I have gotten enough deals from the net that it far offsets paying a pro for good work. Plus I always buy them lunch. :)