For me, if the price is right, the rest does not matter. I'm going to wash everything when I get home so knowing it's prewashed is not an issue, however it might be for someone that normally does not prewash.
What I've seen at private party sales - only one did they cut yardage, but they had a lot of large yardage. Most of the time it's priced per piece and noted - Unless you've got a lot of large yardage (over 5yard chunks) it may be better for your sanity to sell by the piece.
I've bought scraps by the bag - what ever I fit into the bag was $. Well, I was going to but then just offered for the whole scrap box instead (was accepted - box about the size a dryer would fit into). I would not buy scraps in a paper bag, but I have bought them in a clear plastic bag. I think they where like 50 cents a bag - and it was a decent amount of scraps. Again - price has a lot to do with it.
When I was just starting, I went to an estate sale and spent about $500. Filled up the pickup truck bed (have a topper).
I also bought out my Sis's stash at $3 a yard. She only had about 300 yards so it wasn't too hard on the bank. She'd has the fabric in boxes for years and years and we were both very happy with the exchange.