1) You buy it in Had / Hat Yai (flat terrain so the cars there have not has so much scope for abuse - Thai drivers do not know how to drive well generally so areas with hills and many bendy roads are a bad place to buy a car) where second hand car prices are low.
2) Sell it on in Phuket where prices are high when you leave.
THE PROBLEM is cars are registered to a region, so a car bought in Had Yai will almost certainly have that region's plates, which devalues the car's real value in say Phuket. BUT, if you start the process off early enough, you can then register the car in and get plates for another area and the value is not affected there.
I would not buy a car in Phuket because prices are really high (second hand cars sell for almost new car prices - crazy). I would not buy a car in Bangkok because of the stop start nature of the city and the way Thai drivers wait at lights (clutch down, foot brake on).
It is worth doing and I think you might even turn a small profit if you do it right

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