I'd doubt that anyone has ever sold a set like this for $7500 to $8000 dollars. The seller simply lied in an effort to make a buyer feel better about spending $4000(wow, what a deal!) The best way to deal with people like this is either to ignore them completely or politely tell them the truth about the machines value and move along.
I have to disagree with Mark from Arizona as well. The attitudes of the vast majority towards vintage washers and dryers isn't likely to change any time soon, and especially not due to the poor reliability of certain modern machines. After all, if the modern machine needs so much maintenance, just imagine the headaches an older one will cause. Washers and dryers from the 60's and 70's are simply regarded as junk by most, and even if they're in good working order at the time, the next breakdown isn't likely far away - and where will the parts come from to fix it? The ability(and the patience) to track down those parts is not all that common, and patience is needed in spades for many machines. So no, I don't expect to see a jump in vintage washer and dryer prices in the coming months or even years. Sellers like this are just random outliers and don't reflect reality.