What Does "Reasonable Offer" mean?

I find if i am selling something i get offered 1/2, like the set of wheels i was asking 1,300.00 for and got offered 700.00. now if i am buying and seems like i am offering a "reasonable" offer like the tool box that was 100.00 and i offered 75.00, and he said no i have other people interested, ( but i am here and have the $$,,)