In my area, most people having a garage sale are very receptive to negotiating their prices. As a matter of fact, it is usually expected. I was kind of embarrased at first to ask for a better deal, but once you do it a few times it gets easier. There are a few things to remember...
-People having garage/yard sales are usually trying to get rid of stuff they don't need or don't have room for anymore. Use this to your advantage, large items especially are a pain to bring back in at the end of the day.
-Ask things like "Is this the best you can do?" or "Would you take $xx.xx for this?" or "What is the best price you can give me on this?" or "If I buy both of these will you make me a better deal?"
-If you don't want the mattress sets then mention it. They might cut the price and sell them seperately or you could offer to take them anyway and dispose of them for them if they give you a better deal.
-Don't offend the person with a ridiculous offer. For instance, don't offer $5 on something marked $100.
Remember...the worst thing that can happen is that they can say no.
Good luck, let us know how it goes.