No, they're not insane, but they may suffer from another malady: laziness.
They have not taken time to investigate whether the ebay vouchers are a good deal. They have not bothered to price actual meals at Disney, but they have just heard that they're expensive. Yes, food is more expensive at Disney, but you would be hard-pressed to spend more than $10 for most counter-service meals. The few exceptions would be places like Beaches & Cream where their elaborate sundaes run around $7 each.
Don't take my word for it: check out
http://www.allearsnet.com/contents.htm. Add up what you'd typically spend on a counter-service meal; unless you go with the most expensive items on the menu AND dessert, you're not often going to reach $11 per person.
Other foolish bidders may be misled by the sellers who purposefully choose one of the few places where the vouchers are a fantastic deal (like Pepper Market); these sellers list the most expensive items on the menu and show "maximum savings" on their ebay listing. This leads the lazy buyer to believe that the example is typical of meals at Disney when, in fact, it's an extreme example. The misleading sellers also claim that the vouchers include the server's tip; Hello, it's counter-service -- you don't tip at most of these locations!
Either way, it's a case of buyer beware. At $11, these vouchers are an okay deal -- they're not the over-the-top bonanza that the ebay sellers would have you believe. When we used vouchers, we added up our meals before paying and USUALLY we found that we could "save" only if we ordered dessert. Ordering more food than you want/need isn't a good way to save money.