Is there a rule such as used cars over 5-10 years old cant be sold by some/most/all finance companies in Thailand?

There seems to be a lack of second hand finance available

Cars that are bought on finance here are not being sold by the finance companies (unless they are repossessions), they're being sold by car dealers.

 

It is very unlikely that used car dealers would not be able to arrange financing for their own stock.  Financing a car 5-10 years old should not a problem.