I generally try and diagnose the issue, without them knowing what I'm doing. It's not easy.... But I always plan for the worst when negotiating, and have parts priced out in my head before I go.
Quick story:
Ex gf's friend had a decent looking Oliver 1655 tractor for sale for 3k (my price). Went to look at it, and it's in the middle of a field with a plow hooked to it... They thought the trans shit the bed when they were plowing a few days ago and weren't willing to sell it in that condition.... So I popped the shifter off, got it fixed (stuck in 2 gears), drove it around, said I'd take it... "Oh, well... We don't feel comfortable selling it to you since it has a transmission problem... But we do have this 1465 we will sell you for the same price". And that is how I ended up with a turd of an Oliver that there's NO parts availability for, and the Oliver guys don't even classify as an Oliver.