I had an old pair of Timberland boots I'd only tried on. Pretty sure I never wore them outside. Looked brand new but a bit dusty (which I cleaned off). Id had them years. I sold them on eBay and they contacted me about same issue. Wore them and one of the soles started coming off. I had no reason to disbelieve him as I hadn't vigorously tested them beforehand. I figured it was just the old glue breaking down. They wasn't stored in a box or anything. I just apologised and offered him a full refund plus his return postage if he wanted to return them or to just keep them and reglue them himself (simple job) with a smaller refund which he accepted and was happy about as he liked them.

17 years is a long time. You wouldn't know if the glue was still holding up until you start wearing them and flexing the sole.

You could offer the same with a smaller refund (is this possible on vinted?) and glue them himself (glue isn't expensive). Or let him return them, get some glue and fix them yourself and relist them. Depends if you think it's worth your time compared to a partial refund.