The ad does not say "a chance", but says "the chance". While the difference in wording is small, the meaning is significantly different. You say that sgc1999 should have asked for verification that you meant what he thought you meant, but really that is not reasonable when the meaning seems clear. Otherwise every transaction would require an excessive amount of back and forth emails. The limitation should have been explicitly spelled out, and since it wasn't, you should honer the request for a refund or offer a reduction in price.
Just $.02 from a new guy.