If ypur buying a bike off ebay be sure to check the wheels are true and not buckled. If not this can blow your budget sky high if you have to buy a decent set of wheels.Check the headset isnt creaking when you rock the bike on the front brake.Also make sure the seat post isnt seized in the frame. Just ask to alter the height of it.Look the frame over to see if it isnt bent, daft i know but lots of ebay bikes have been in crashes.The groupset, gears will either work or not.Finally grab one of the cranks and rock it to see if there is play in the bottom bracket. If there is movement this could be expensive if your not mechanically minded or dont have the tools even for basic bottom brackets.Good luck.