Hello, and welcome to the forum.
As for selling gear - I have had very little experience of places like gumtree or ebay - and of that, most of it was bad.
On this site, I have even less experience - only having bought one bass - but the process was very painless, and very rewarding.
If you are selling, I would make a point of reading the rules - in particular the one about including a price!!! (methods of payment accepted could also be useful) - It would also help to include a good description and some photos ( there are some threads pinned in some of the sub-forums about how to add photos to your posts), the clearer the better..............Oh! and it would help if you included your location, at least the general area - especially if you are wanting pick-up only, cash on collection, or offering to let people try before they buy.
I'm sure some-one else will be along to offer better/further advice on buying, and selling - but for now.....have fun.