Yeah i totally agree with 02GF74
If you are on a Budget with only £200 at your disposal then
Ebay is definitely the place to look, It does help IF YOU KNOW WHAT IS GOOD AND WHAT IS NOT SO GOOD....
If you are thinking of components,ie SHIFTERS/GEARS/DERAILERS then they start low end (price wise and quality) with Acerra (cheap) STX (Old Skool) but better than Acerra,(DX Which was the poor mans LX but Very good quality and pretty tough,(LX Entry level, good quality and lasts) (XT very good ) & (XTR Excellent Top of the range)
This is just a quick rundown of the components that most second hand bikes on Ebay will have, But prices will vary with the upper end components.
Another option is to purchase a Second hand frame from Ebay, and i have seen some rather cheap frames ie, GT,KONA,CANNONDALE,TREK, Or the BRAND NEW alloy ones that are on there now for £49 BUY IT NOW, And for £50-£60 with delivery for a Aluminium frame is pretty good then you can kit it out with components of your choice.
Like Weinman Rims with Shimmano Disc Hubs ( F & R ) £64.00 BUY IT NOW, You could probably build a complete bike for £200 from buying off Ebay as it all depends who wants it at the time its advertised.
I have picked up some real bargains on Ebay over the years so definitely the place to look if you are on a tight budget...
hope this helps
Chris