I can't afford to buy what is a wonderful lens (even second hand on ebay). I don't know whether you shoot monochrome or colour but I suspect that the best thing is probably to quickly shoot off a roll of film and get it developed ASAP and then produce some quite large selective enelargements to see if you can spot any flaws. If you can't, keep the lens, and if you can, then get it sent straight back to Keh.com.
I recently hired a 30mm lens from Calumet in the UK and the ND spot filter that came with it was, to say the least, a little scratched from previous heavy handed use. My 16 x 12 prints seem to have come out ok and it might be the same with the internal paint flecks within the lens?