If you have a number of physical photographs supplied by the photographer, then there is nothing to stop you continuing to sell them on, since this is covered by the doctrine of exhaustion of rights
However if you just have a digital file which you would normally either copy digitally or print from in order to supply your customers, it will all depend on the licence or agreement you had with the photographer in the first place. As you are asking the question, I'm guessing that nothing was actually said - either verbally or in writing - about the length of term of the licence. A licence is a form of contract and so the normal rules of contract will apply. This means that if a court were to have to decide whether your particular contract with the photographer continued to operate in the eventuality of him selling the copyright, they would try to establish what the two parties (ie you and the photographer) intended to happen at the time the contract was made. Clearly the best evidence would be an existing written agreement plus verbal submissions from you both, or if the agreement was verbal to start with, then just your submissions about what you thought had been agreed. The court would look at the issue from the point of view of what a reasonably well-informed 'ordinary' person would have thought the licence to mean. And of course although none of this involves the new owner of the copyright, it might be relevant to examine the document which assigned the copyright to him/her. Transfer of ownership has to be in writing. If this assignment mentioned any existing licences or stated that the assignment came free of any such licences, then this would tend to weigh in your favour, since the photographer would at least have had to consider your position and contact you in advance to say he was cancelling the licence if that was indeed what he intended. I assume he didn't do this and so you have good grounds for believing the licence continues to operate.
But a second important contractual factor is whether you were paying the photographer a royalty to operate the licence. Did any money change hands at any stage, and if this was supposed to be an ongoing arrangement, how were the royalties calculated - on a time basis or per sale made? If there were no payments at all to the photographer, this greatly undermines your case, because a contract requires that there is benefit (known in law as a 'consideration') to both parties to the agreement. Clearly if the photographer was gaining no benefit from the arrangement, then this tends to suggest he intended it to be a one-off or short term arrangement, because most contracts such as copyright licences are there to be part of a commercial process. Conversely if the photographer was supposed to receive a percentage share of each sale then this strengthens your case for assuming that the licence was, at best, open ended and therefore continues to be valid now.
As it is highly unlikely that the matter would ever go anywhere near a court, it may require some independent arbitrator to follow a similar process to establish where you stand. Have you actually contacted the photographer about this? I presume you haven't or if you have, he has not supported your case.
So, as you can see, some more detail is required before we can advise on what you should do next. In the meantime I suggest you do suspend any advertising in order not to inflame this situation. And if you are already in communication with the new owner, it would be sensible to inform him/her that pausing your selling operation is out of courtesy, and does not in any way indicate that you are renouncing your claim to a valid licence.