Firstly check the market and then synchronize it with your budget. only for used mobiles!
And then you'll have some thing in your mind.
Always check Camera while buying HTCmobiles, usually they din't have good cam, But the sound quality is always good. Atleast go for 3.2MP cam never go for something below than that, coz these are such dark times(with 2MP cma you'll stand in the line primitive people from african jungles )!
Secondly Check the Wifi's(if the phone has wifi).
Check the OS version, try getting a phone withWM6 atleast or you'll be disturbeD! ( you can falsh it afterwards to a higher OS verion easily, without any problem).
Other details are not much important.