If I can add something to my quick answer. I just looked at the design patent invoked. It seems this gentleman only has rights in the US and nowhere else. I clicked on:
OVERVIEW OF PATENT CASES FOR THE FAMILY OF USD722681(S1)
and saw there was one case in the US (which I'm not signed up to look at), but, for obvious reasons nowhere else. What the implications are I cannot say, but he could only sue for infringement on the territory of the US.
Rest of my answer unchanged:
I had a quick look at this and don't know many details but I imagine you received a complaint letter from the holder of the design patent. If it was from his attorney, preferably an attorney should reply. I can't imagine it was a summons before court.
In each case, the approaches is the same. You say you are a third party vendor - a quick look at the webpage at the link you provided shows this. You are not however the true infringer. The infringer is the producer (who is perhaps located in a country where IP rights are not always well respected) of this fine looking object which you were, perhaps, selling ( it is marked something like not currently available ).
Now, it is difficult to imagine that you are the only person in the world selling this object. It is quite possible you have received a standard letter from, I imagine, the holder of the design patent which he sends each time he sees something he thinks might be infringing his rights, and he may have been doing this for years.
The reason he sent a complaint to you (assuming it came from him) is that your offer of sale is extremely visible and it is easy for him to keep a lookout for anything he thinks is infringing his rights. But what he is really looking for is the identification of the producer, who is the infringer and from whom he could possibly obtain damages. Your webpage is just the ripple on the surface which might allow him to catch the big fish. Perhaps you know who the producer is.
What the owner of the rights could (or should) want to know from you is how many of these you have sold (if none, that would be ideal), how many you possibly have in stock and, importantly, who is your supplier. He would also want you to give an assurance that you have taken down the webpage that shows the offending object, that you have stopped selling, or offering for sale the object and with an undertaking you will never start selling it again. This should not be too much of a loss to you. After receiving a well written letter (which should be drafted by an attorney but which could be sent by you - the earlier advice to seek legal advice from an attorney holds 100%) he could well leave you alone and attack your supplier who would then ask to be held harmless by the actual producer. The latter is the infringer, and it is up to him to argue how his product differs from that of the holder of the design patent or that the design patent is invalid. Don't do his job for him ! and don't be tempted to reply with arguments which could not be beneficial to you. If you try to negate infringement, he won't let you off the hook, and things could get more complicated.
I don't know the facts here but these are the general principles and I hope some of the above is relevant to you.