It depends on why you are buying/collecting the old quilts. Are you buying: to use on the bed; display for decoration; historical interest in fabrics or patterns and or (The Sky might fall on me with this one) - to cut up and make new items from them. I don't believe that every old quilt needs to be preserved and saved and they have uses beyond the original purpose.
So decide how you plan to use the quilt and then decide on what you will collect and the price you are willing to pay for them.