I think that if you knowingly promote an item which infringes trademarks or copyright then you might be vulnerable in law.
The originator of the dodgy design, using someone else’s copyright without licence, or unauthorised use of trademarks or infringing trademarks is the prime culprit, and as such is subject to legal action directly by the trademark holder/copyright holder and their lawyers.
I am not a lawyer, but from my experience so much law is about ‘…knowingly…’ acting in a certain way. A customer buying an item at a craft fair or in a shop where there is an infringement seems, in my opinion, to be protected from action assuming it is for their personal use, or to give as a gift to someone.
However, even the suspect goods, bought by a member of the public, could be confiscated and destroyed if the originator of the design or trademark wished to go down that route, and had proper evidence.
However, a trader or dealer buying such items , particularly in bulk exposes themselves in law, I believe, as not only will those goods en masse be confiscated and destroyed if found out, but they could be done with heavy fines by trading standards, and the original designer too, through the courts.
Just as in pollution law, the polluter pays…, the trademark holder or Trading standards will want to go for the source of the infringement first, then after the dealers who try making money out of the sale of goods. But we have to put this into context, and a lot depends upon the profile of the item or goods, and the scale of the infringement.
It is almost impossible for designers to completely control this, and many top designers employ lawyers / investigators to search the shops and the web constantly for infringements. The smaller designer cannot afford to, or hasn’t got the time.