I don't actively offer or advertise a warranty, but if someone who bought a bike from me was unhappy later I would definitely work with them to make it right or refund their purchase. This is not something I've had to deal with though, because I would be terribly embarrassed if someone who bought a bike from me found that I had done shoddy work, so I am extremely careful about having the bikes I sell in impeccable condition, and I make a point to show potential buyers, in detail, anything on a bike that's less-than-perfect before I take payment.