And if you do any work for them, get the full amount, plus some, upfront... I once did a pre-purchase inspection on a nice C90A in South Africa for a wealthy Indian Kingfisher Airlines 'Captain'. Agreed work scope and price, 50% upfront as I usually do. Big mistake. Not cheap to travel there, review the logs, the aircraft and do a couple HSI's
Me - It's a good plane, I have the sale documents ready to go, you've paid a deposit, let's conclude the deal
Kingfisher Captain - No, I don't really like the color of the stripes
Me - Ok, no problems, your money, your choice, but you've paid a deposit, it really is a tidy plane and a good deal, we can paint out the striping
KC - No, I don't want it now
Me - Ok, no probs, here's an invoice for the balance owing
KC - But I didn't buy the plane?
Me - I know, but I completed the work we agreed on, for the amount we agreed on
KC - But I didn't buy the plane? Why do I have to pay this?
Me - Because I did the work. I actually traveled to SA and physically performed the work we agreed on for the price we agreed on
KC - But I didn't buy the plane so I don't have to pay?
...and repeat for months on end until I gave up and vowed to never deal with one again.