Had a scan through their T&Cs on their website, and on quick glance it appears to be very silent on the issue. There's stuff on there about their policy towards stolen items and the efforts they go to to seek to prevent items being offered for sale that are stolen (particularly mobile phones). But at the end of the day, it seems that there's nothing more they offer.
I don't use the place, but personally i wouldn't, unless Cash Converters contained a clause in the buying agreement that indemnified me in the event that an item bought turns out to be stolen. In other words I think it should be their risk, not mine - that's what they should be offering.
Perhaps surprising (in the light of above and the scale of this operation in the UK) that Trading Standards haven't latched onto this.