Anyone can ask anything they want for a cover; if it remains unsold it's the seller's problem that he overpriced the item. In fact, don't I see a "40c" marking in pencil under the cachet?
Given the age and scarcity of the cachet, it may be worth perhaps $5 or $10 on the secondary market (several are for sale at that price point) but I would agree $30 certainly seems overpriced for the item. Of course, if one is intent on obtaining "unofficial" postmarks, such as the one shown, a premium value may be justified, but in the case of Scott 798, there are more than a couple of dozen "unofficial" postmarks known for that issue, with League Island being one of them.
I'm not sure of the scarcity of the League Island postmark, though, although since the postmark isn't "attached" to the stamps, I doubt it would merit that much of a premium.