You can do classifieds on qs but the site is barely alive so you wouldn't be in much danger of selling anything there.

If you list on ebay and gumtree, I'd say nothing about listed elsewhere, but if a gumtree buyer wants it I'd tell them you have to end the ebay listing before you can let them have it.  As far as ebay knows, you could have been talking to your neighbour and they decided they wanted it.