Originally Posted by contango

Hard to say where to look with that short a window. You could try ebay and restricting your search to bikes close enough to go and collect. Or if you've got a local bike shop they may have something available - my LBS doesn't do second hand bikes as a rule but every once in a while they'll take something in as part exchange against a new bike and when that happens they have a used bike to sell.

If you're looking at spending £50-100 then the chances are as long as what you're looking at doesn't leave you thinking "all this for £50?" you'll probably be OK. Realistically at that end of the market the chances of the seller having their original receipt are pretty slim - my bikes would probably be worth £3-500 or so as used bikes and I don't know that I've still got receipts for any of them because they all date back to 2009 and are well out of warranty. Unfortunately the chances of picking up a stolen bike are one of the risks you have to take whenever you buy used, although if you can get something from a bike shop or a cooperative or something you'd hope they had done basic due diligence to avoid reselling stolen goods.