Idk if you collect (or know anyone to collect) sports cards, but Panini, Upper Deck, and Topps will short-print cards to a given print run. The term "eBay 1/1" refers to a card whose short-print serial number matches the featured player's jersey number. While it's not a true "1/1" (which is a print run of 1 card), it's a marketing device that is a sort of unique card because there will only be one jersey number in the print run.
I feel like these jars you're referring to are similar to that. People were far more religious in those days, and thus very superstitious, so I could see the superstition against the number 13 being a fact. As to rarity, I don't think they're particularly rare (that I'm aware of) and the high sell price is based less on actual supply and moreso on the historical superstition. If you're interested in selling them, I would do the pricing research and then sell them one by one (keeping supply low) for as much as you can get. Hope this helps...