Yes a customer should be informed of any "Known Issues". I certainly wouldn't tell this customer about it now!
Yes the repairs should have been paid for by you.
Every lesson does indeed have a cost. Thank God that your cost is Wholesale. $4,200 is probably $1,500-$1,800 job cost.
Where these things get tricky for me...if the $4,200 had been done up front, how much more would you have sold the vehicle for? In this case, probably no more because the guy pricing the unit is pricing to Market, not from Cost (I'm guessing). So the REAL question that I invariably ask myself, should I have Wholesaled this unit? Who Appraised it as a Trade (if it was a trade). Could we have Arbitrated if an Auction unit?
Either way Bill, make damn sure this customer knows that you were in no way obligated to make this problem go away BEYOND the appreciation and respect that you have for your customers. I'd probably even arm twist a play by play 5 Star Review. These reviews are absolutely priceless!
Know this Bill, you've got company on this hill. If you die on it, I'll be right there with you. We can die together. I'll bring snacks.