How is a seller supposed to know if someone owns the sole rights for a particular item?....

My opinion is that once a person has paid money for a product - that person owns the sole rights to that particular item.  

The manufacturer openly sells goods on the market so they can make a profit.  I fail to see what right they think they have to then tell the purchaser what they are not allowed to do in the future with a product the manufacturer no longer owns.

If it was me in OP's situation, I would be telling that manufacturer to give me a 100% refund on my purchase, if they refuse to allow me to sell my property on the secondary market.   I have a major aversion to anyone telling what I can/can't do with something I have bought and paid for.