Did you join WORM?
To be honest you'll be lucky to find anything worth having or in a fit state to repair for £500. Some reasonable bikes can be found if you shop around but eBay can be a minefield of junk. To go green-leaning it'll need to be road-legal for use on byways, which makes your search that much more difficult. Looking on pre-loved and Gumtree may yield something and the best classifieds will be in the specialist off-road press such as Trials & Motocross News (if that still exists) or TBM (which definitely does still exist) - look in somewhere like a big WH Smith to find those titles. Happy hunting
