For me i don't normally mention abt any personal warrenty... as i check my stuff ok before sales.... and allows my buyer to check on spot.... but if within the next few days any problem occurs... i will try to solve for my buyer if its not due to buyer's careless.... but i do get pple calling me back after almost a month later( lucky non from this forum yet n its from other forum).
I think to be fair to everyone... check on the spot.... even if anymore problem...its should be within a week....
It's about providing the warranty for the internal operation. It's for the benefit of the buyer. If the buyer never asks for it, don't say you'll provide the personal warranty.
Externally, if after purchase/trade/deal & you're not satisfied with the physical appearance, then there's no way that it can be returned. Because the appearance is "as it is" & can be seen before selling.