depends how nice you are and their track records. if indeed they found problems with the item they are doing you a favor. if you cancel, leave a note saying seller requested this. if not you can wait until the deadline is up or they forcedfully send the problematic item to you and you have to issie a return and refund.
In that case, i suspect they didn't fulfill my purchase as they were able to sell the exact same item for a lot more in the following days. Gladly, i was lucky enough to find another comparable listing soon after and item arrived safely home this week.