i've bought things through BPL that have had X's on them. they were priced right, and in great shape. if the guy was able to pick them up a little cheaper than what he sold them for, great for him. if he looses a little money, thats his decision. either way, its NOT your job to call him out for it.
i love the REI garage sales, and i always end up picking up something when i go, and if for some reason it doesn't fit or work like i might want it, i should have every right to try to resell it at whatever price seems fit. if you don't like that price, don't buy it. just because it has a little x on it doesn't mean its worth any less than one without.
if we really want to find something we all do not like, its the people who go into the garage sale with a basket or something, start putting everything in the basket that they can, and then sort though it later to see what they actually want. that's what burns me, but its impossible to avoid....