As IRL, I don't hesitate to ask for a discount when purchasing in bulk, or if I am setting up a new lot and am buying a HUGE number of items at once. I might also speak to a store owner about hooking me up with all the items I need for a lot at a discount over the period of a week or whatever, at a 55% discount. That 5% adds up quickly and can make the difference between a lux finish and meh. We all know the actual price of the finite items available, so what you are really negotiating is the time and effort of the store keeper. If you have a list and are ready to purchase on the spot, it's minimal effort. Only difference for me is the crafted items. Not 100% success rate and the investment is pretty high. Those I just shop around for the best price. Same with the clothing, just shop it. If no discount is negotiated, I'd look for a better price. but ALWAYS be polite. No one OWES you a deeper discount.
NOTE: All negotiations should be done privately in PM!!! NOT in open property chat!
ETA: I don't nor have I ever owned or roomed at a store in FSO.