I've bought a bit of stuff (inlcuding my current car) off Gumtree, but there's no way I'd ever sell on there!

Gumtree doesn't cater to the same type of sellers as eBay - if you're trying for volume sales, Gumtree's just not the place to go. If you're hoping to list for $5 opening price, sell for $25 final price, Gumtree's a dead loss.

From my own POV, the problems with Gumtree are:

1) Woeful lack of search functionality - it's just awful. Key-word searching is a nightmare: if you enter "leather jacket", you'll get every listing that has either of the two words. Youll get leather gloves, denim jackets, etc. The leather jackets you were actually searching for will be in there somewhere, but buried amongst a whole pile of other stuff that you're not interested in...

2) They're not "auctions", they're effectivel "fixed price" listings  - if you list an item at $10, that's what it'll sell for. Fine if you were only ever doing "But It Now" listings on eBay, but you'll have no more of the old "two buyers going nuts bidding against each other" and driving the price up - opening price is final price.

3) A lot of Gumtree users search only in their local area rather than nationally - so, many of them will never see your listings. Again, useless if you're going for volume - you want your listings seen by the maximum number of users possible.

As a sidelight, I've lost count of how many eBay listings I see with a version of "I reserve the right to end this auction at any time, as the item is listed elsewhere" spiel, particularly on higher-dollar items: cars, motor-bikes, etc.