My personal tip, would be go ebay and then set the search for who lives closest to you if say, you are looking for a bicycle. Then, you can bid, save usually on the shipping cost which is a big advantage by picking it up. With ebay too, people get feedback, it's not as much of a blind transaction as with gumtree. You doubt someone's going to rip you off, if they've got 1000 or a 100 positive feedback reviews.
I agree, there can be a bit of apprehension on lists like gumtree. Sometimes people run ads for bike co-ops and things like that, so that isn't as bad. I would never buy something stolen.
Admittedly, one can't always buy from someone close by as far as ebay goes and some people live far from the cities, ebay can still be of good use. I bought something from someone about a 90 minute drive away but even the shipping was reasonably inexpensive. Honest seller they were. Some shipping costs are pumped up inordinately.
