I've found similar pricing on several books I've looked for, although I'll admit the $6,000 is a new high!
These have always been rather obscure specialized books
I've always assumed it was either a typo or someone looking for a "lottery like" sale.
I suppose one could always contact the seller and ask if its a mistake.
I've never bothered because I've always managed to find the books I was looking for at a more realistic price.