Ive bought 2nd hand cars with number-plates from Bangkok, Chang Mai and even Trang.....If you want, you can change it to Phuket-plates, but I was told from the registration-office in Phuket that I dident have to do it.
Anyway, my advice is to change it. It seems like the police is more into stoping cars from othe provinces. (my Ttrang-registrated car was very inn at the check-points
)
About the fixed prices, the weekly big car-magasines from Bangkok that you can buy in any bookshop, normaly have pricelists for new and 2nd hand cars.
As far as I can see its a good business-opertunety to buy 2nd hand cars in Bangkok and sell them in Phuket. Seen 250-300k cars in BK being adverticed in Phuket for 350-400k..............
Geir