there are always hundreds of ex hire cars for sale in ibiza at the end of every season. our very own james bought his this way and is very pleased. there are also second hand car dealers on ibiza if you're looking for something a bit older and cheaper. having said that i bought my first second hand car from a man in barcelona - the market is larger so the cars tend to be a bit cheaper!
i don't think it makes much difference where and how you buy - just like in the uk!