Search for Money Saving expert - they have a phone recycling comparison service.
eBay is only as trsutworthy as the person you are dealing with. If worried, sell it as a classified ad for £140 with best offer. You can fine tune it to accept cerrtain offers and decline others.
Depends on condition really, my phones tend to be well loved come renewal time so I go with the ease of a phone recycler.
I'd trust both as much as each other and some companies will try everything to pay you less for the phone I am sure. (No personal experience of this though - you'll be able to find reviews on them all with a quick search.)