Two things. 1.If the guy had a similar bass with a bad neck and he wanted to swap it why wouldn't he just do that right away vs waiting so long? 2. Everyone is going on the idea that the buyer is a scammer (and maybe he is) but what if a legit neck problem popped up? No neck no matter how it's made is guaranteed to be perfect for life. Or maybe he threw super heavy strings on and doesn't know about adjusting the truss rod. Be polite and ask that he take it to a tech to provide you with info.
Even if a neck issue popped up after a month, it’s not the seller’s responsibility. There isn’t a warranty when you sell something used, which is why the buyer was able to buy the bass at a $1000 discount. I sold an amp once that a buyer contacted me after two and a half years saying that it died. I didn’t feel bad ignoring that message.