In addition to the reasons already given, some others may include:
Sellers who don't pay monthly for a store cannot put their items on hold when going on vacation (holidays). So, setting an usually high price is one way to temporarily suspend sales. It's still annoying if you have many active items.
Some sell other places in addition to eBay (e.g., other sites including their own, utilizing eBay for the transaction handling) and may be able to sell items to their own customers at a higher markup due to additional support, etc. Of course, this wouldn't be a likely explanation for outrageously high prices, but for ones that tend to be on the upper end of the market's spectrum.
Not everyone who buys on eBay necessarily does their homework. Some sellers may try to capitalize on such uninformed buyers or ones that may be desperately in need of some item. When all the items at reasonable prices have been sold, all that's left are ones that are unreasonably priced.
Of course, some sellers don't do their homework either and list items at their desired price, not what the market is likely to pay.