I don't know much about gumtree but I'd assume you can stipulate cash on collection only. From other threads I've read, there's no paypal protection for buyer or seller on gumtree, but if they pay by paypal and don't pick it up you could run into difficulties if you've still got the items when you're ready to move.

I'm not sure about ebay but I'd assume paypal is compulsory. There'd be nothing wrong with saying "cash on collection PLEASE". I'd have something at the very top of the description saying the items have to be picked up within X number of days, and give yourself plenty of spare time.