A level 6 cub on ebay? You mean they are trying to sell an ingame pet that's lvl 6? If that's the case, be aware that selling ingame items for RL cash is against ToS, even though selling the unscratched loot card is perfectly OK.
As far as lvl 6 cubs, it seems pretty likely that they're duped. Bliz has not stated any position on the cubs, and they've only very obliquely admitted that they're aware of it. In the past, they have never taken these sorts of suspicious items away from end buyers (except in extremely rare circumstances where people bought many many copies). There is also no unique identifier associated with pets in the pet journal or in cages (at least that is visible to the client), so it's questionable whether Bliz could actually identify which cubs were dupes if they wanted to. So there's very little risk associated with buying a 'duped' cub versus a 'real' one. The main risk is buying an ingame item on eBay, and also the risk that cubs will continue to decline in value. They're currently in the 60-70k range on high pop servers and haven't dropped a lot in the last 2 weeks or so. But since we have no information on whether the duping has been addressed, there's absolutely no guarantee that they'll stay at that price. The only good bet is that they won't go back up.