I basically sold a hw100 for about £100 under what I reckon I could of got for it as it was leaking a tiny bit. I could of sent it off (as I don't have the skills) waited the (up to) 6 week turn around and with postage covered both ways, roughly get charged £100 or more as I personally don't know anyone to offer it too other than a Google search.
High end or collectable, I'd fix them sell, low end it's a case of if you have the time to wait for a repair
Never try and offer px with a leaking gun, I was on the market for a ultra and got offered (about 3 years ago) i think £100 for my fx. Hard no obviously. Got it fixed and it's still more accurate than anything I've bought since or before
Super soaker 3000 (water), nerf fang (foam), noisy cricket (energy), m41a pulse rifle (10x24), Gat gun (.177)