This has been really hard to track down, so any help appreciated. I found a story in an anthology of fantasy or science fiction about a shop that advertises an odd exchange program: for ten (?) of any item, you get one - only better. The protagonist exchanges ten pennies for one which turns out to be rare; then ten cigarettes for one which is almost hallucinogenic, it's so good. He doesn't trust the woman who runs the shop, suspecting that she's running some sort of con. I remember that the story was called "ten for one", but I suspect that may be wrong, given how hard its been to find.
This Offer Expires by Liz Hufford
The old store on Lesort Street had a sign in the window - TWELVE FOR ONE.
The protagonist assumes that this means twelve for one dollar, but the deal is explained to him:
"Simple," the woman said, "You bring us twelve items and we give you one."
His curiousity aroused, he trades 12 cigarettes:
Once it was lit, he forced himself to take a substantial puff. It was marvellous. A well-defined but mild flavor, a wonderful aroma. He took another drag. He was very relaxed. The room became pastel; his vision came in swirls. He quickly put the cigarette out.
He goes back, accusing the woman of peddling narcotics. She tells him that it won't test for anything. Next he pulls twelve pennies out of his pocket and gets one in exchange.
Then he held it up to the light. It was a misprint; there was a double image of Lincoln.
He takes it to a coin dealer:
The man looked at the coin through a magnifying glass and offered him a hundred dollars.
(I won't spoil the ending.)