I wouldnt pay scalper prices for them but if he can comfortably mine with them if the price drops 30-60% and can mine for 18-22 months without any financial stress then it should be okay. I personally always look at the current ROI and multiply it by 2. If I don't mind running them for that long and it won't cause me any additional financial stress then I would do it.
Find the cards with 36 months warranty, it becomes a free lease of the cards to mine with if they break under warranty esp near the end and then return when faulty for refund or credit towards new ones.
Regarding the warranty.
There are some companies, Sapphire for one, where you need to be the original owner to get any warranty what so ever. They ask for proof of purchase and it needs to be purchased by you, so even if you get bill of sale from the scalper, it might not help you on the warranty concerns.
A few years back, for Sapphire you had to mail it somewhere overseas. No idea if they got a USA address these days. But I remember the shipping with tracking was extremely expensive and extremely slow. So it wasn't worth sending it in for RMA if it had minor issues such as bad fans. You would only use the warranty if the card was dead and unfixable.