I've used Gumtree numerous times, and AFAIK you 'buy as seen' from a private seller. I think it is up to the buyer to make sure it is in working order or all parts are there.
Saying that, I think you can take it to a small claims court but do you think it will be worth it? The time and expense?
Anyway I found this explaining your rights as a buyer of second hand goods from a private seller.
*If you’re buying from an individual – which constitutes a private sale – the rules are slightly different. For example, the so-called ‘implied terms’ of the Sales of Goods Act only apply to title and description, not to quality. This means the goods must simply correspond with the description, and be legally owned by the seller.
“That means a dress can’t be a size 12 if it was described as a size 18,” says Stephen McGlade, a solicitor at consumer group Which?.
However, if an item is advertised as “a three-year-old bike”, for example, it doesn’t mean it has to work, just that it has to be three years old. In this case, especially when the item has been well-used, the transaction remains a case of caveat emptor, or ‘buyer beware’. “This, put simply, is why you pay a lower price for second-hand goods,” says McGlade.
Getting your money back
If the private seller has failed on any of the relevant parts of the Sales of Goods Act, it is up to the buyer to take issue with them, quoting that they have contravened Section 13 of the Act. If they do not rectify the problem by way of a refund or replacement, the next port of call is the small claims court. However, after you factor in legal fees and the time and hassle of a court case, this may not be worth your while.
If you’ve bought an item from a flea market or car boot sale, you may not even be able to trace the seller in the first place, so these purchases really boil down to an even clearer case of caveat emptor, says McGlade. “As the trader is not an official company, any purchase will be regarded as a private sale, which gives you limited rights – if they can be traced at all. In this case, it’s really up to the buyer to exercise caution.*