Signature and cachet question

[pbspelly]

So I saw this for sale on eBay (https://www.ebay.com/itm/BILLY-SULLI...IAAOSwBjdaOz0d) and was wondering a) if you guys think the signature is real and (b) what exactly is a "cachet" and how do they get date stamped? The reason I ask is that I am almost 100% certain that Sullivan (my grandfather) was not in Cooperstown in February 1991, and I am 100% sure that he was not in Cooperstown July 31 1994, since he died earlier that year. On the other hand, it does look like his signature and hand writing, and it would be very like him to write the years that he played for certain clubs. I also kind of doubt it's fake simply because it seems like a lot of work for someone to go through for something you're selling for $7.99. But I am just not familiar with cachets and how they get stamped. It doesn't have an address so it couldn't have been sent through the mail as a postcard. Any insight?