I wouldn't buy them second hand unless you can inspect them, particularly the condition of the teeth first, and make sure you stay away from the old version HKS gears which were notorious for tooth wear which led to timing belts slipping - not something you want!
If you have the money, Tomei and HKS both make their new version gears from Duralumin and are the pick of the bunch.
For something a little more budget friendly, get yourself a set of OS Giken gears, they are a fair bit cheaper than both Tomei and HKS and I have never come across anyone who has had an issue with them