I think with the TB's you choose a set aiming towards something with the sort of power output you are after.eg gsxr1000- 170 bhp etc.
Although i stand to be corrected.
As regards to Ebay,i have never really had any probs with either uk or oz one.The best advice i can give is to buy from a seller that has a good feedback rating (98 % or more) and/or also has a online shop selling such parts as they will want to keep their good record.
In the event it goes bad most transactions are covered by paypal buyer protection if you use paypal.(Definately worth doing)