I have tried to bargain on prices, but never had any luck. I ask for freebies like cables and strings when I buy. I think you have a better chance with the used gear. I live by 3 GC and they always have old Marshall heads. When I get some cash I am going to make a low ball offer. I have in the pass tried to sell gear and shit, they want to give you practically nothing. Something like 30% and a little higher for store credit.
GC hires more by looks it seems. In my experience over the last 26 years, any item you are willing to buy, the sales guy will let you know what you are buying is the best. I once asked about the Zakk Wylde overdrive when it first came out. The salesman told me it was the best overdrive out there and if I purchase one, I would not regret it. Well I did and I did.