It depends on the buyer and the type of material they are selling. For CPU's, RAM, and larger quantities of boards, I usually buy by the pound and the sellers are usually quite happy with that. If the seller gives me a box of mixed scrap parts, I just give them a price on the lot. You will need a bit of experience to make a quick estimate on the scrap value of a mixed lot. For whole computers I always buy by the piece. Some sellers do not want to bother with weighing the items, others very much prefer to do so. I also round up, it helps build trust, and the seller feels like they are getting a fair deal. I always make clear before going to the seller, that I'm buying the parts for scrap, not for re-use, and that I pay $x.xx per pound or per piece. That way there is no misunderstanding, and I don't come home empty handed.
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