I am not an attorney, what I offer is only opinion:
1) Stop payment on check ASAP
2) Contact seller and tell him you are stopping payment and returning merchandise
3) Send Certified RRR letter to seller outlining that at time of purchase you had specified no older than 6 years and since you have found that the merchandise is older that you are returning stuff and stopping payment. (this is to document what you are doing, in case you have to go to court on this)
4) What exactly is specified on invoice/contract? Is there any return policy stated, stipulations, etc?
5) Wait and see what happens: 1) he can take you to court
or 2) he is out of luck. (since hopefully you stopped payment in time.)
Please keep us posted.