"Should curses be available for purchase ?" -
The most common scenario for purchasing curses (in a game where they aren't otherwise gained) is in a two-player game where buying all (at most) ten of them would end the game, resulting in a win for the player who does so. Usually this requires some/all of the following:
Watchtower in hand, so you can trash them immediately. (Note: I was about to put Trader here also, but that doesn't trash the curses; you just don't get a curse...)
Talisman in hand, so buying a curse gets you an extra.
This ("scooping" the game) frequently happens when one or both players in a two-player game are buying-up all of a few kingdom cards (e.g. Fishing Village, Bridge, City).