You'll find a few approved traders in our directory, Via is just one of them Link hidden: Login to view
A liquidator sells surplus (liquidated) stock, so that can include items such as shelf pulls from retailers, customer returns, closing down sales and so on. So you're talking about a whole range of items in all sorts of conditions, so you do really need to be aware of what you are buying because if you aren't aware, it's an easy market to get yourself burnt in.
Mobile phones probably aren't all that common in the surplus market, but you never know what you can turn up. Another option might be liquidation.com, it's not a genuine liquidation centre like Via, but you can often buy lots of mobile phones there. It's an auction platform and quite often you are buying cherry picked lots, so again, you need to be careful.
By cheery picked, I mean someone has already purchased a larger lot of items, then taken the best quality for resale, now dumping what they don't want in a bulk lot to recoup some of their initial outlay.