Sure, I'd let that stand. (Of course, it would have to be a truly remarkable Bluff roll - the Bluff is pretty hard to believe, and so I'd award the merchant a +10 or even +20 bonus on the Sense Motive roll - per the PHB p.68)
Of course, the characters then have to deal with the consequences of their actions:
1) There's a probable alignment violate here for any Good or Lawful PC - they've just robbed an (assumed) innocent of a hugely valuable item. Certainly, this is an Evil action - hurting others for one's own gain. It's also a Chaotic action, as an expression of personal freedom at the cost of trustworthiness.
2) The merchant obviously didn't
Identify the potion they sold him. Did the PC's
Identify the ring they bought in turn? Who's to say they didn't get ripped off?
3) The con
will eventually be discovered. This then leads to all sorts of fun opportunities. If the merchant was at all reputable, the PCs have just sullied their reputations in the town where the trade took place, and are unlikely to be able to buy any goods and services there except in hard cash, and probably with a hefty mark-up, if at all.
If the merchant was not reputable, of course, he most likely has access to the underworld, and is likely to be sending bounty hunters/assassins/debt collectors after the PCs.
And bear in mind that any merchant who deals in magical rings must have some considerable clout, or else he'd quickly be robbed and/or dead. Frankly, the PCs are likely soon going to be wishing they'd just paid the asking price in the first place.