The owner may be accepting cash as lot payment and may not be reporting it on his taxes. You may want to ask for the bank deposit slips and/or his monthly bank statements to check deposits.
If the owner’s asking price is based on gross income of $123k, definitely make him prove that’s what’s actually being collected.
I would make a offer based on the tax report of $75,000 and tell him he will need to prove otherwise if he wants a better offer.
Put the ball in his court and make him prove it.