The Dominion rules contain the following note:
Note:
Curse cards are present in every game, however, they are rarely used in the basic game other than with the Witch card.
I believe you can buy them just like any other card.
The question is, why anyone in their right mind would ever want to!
Just to show off, I guess :D
boardgames.stackexchange.com/users/436/thesunneversets
There are some subtle situations where you need to have curses on the table even without an explicit curse kingdom card.
For example, the Swindler from the Intrigue expansion lets you trash opponents cards and replace them with cards of equal value.
If you're trashing a zero-value copper you
can replace it with a zero-value curse .
Without curses all you could do is replace it with another copper.
boardgames.stackexchange.com/users/675/Adam Wuerl
When playing with "Fairgrounds" from the Cornucopia expansion I sometimes buy a curse just to have another type of card in play.
boardgames.stackexchange.com/users/2156/Gildavinor
Friend of mine bought one in a Garden-based deck - he had 79 cards and copper ran out.
boardgames.stackexchange.com/users/2098/Reactormonk
"Should curses be available for purchase ?" - Yes!
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.
Multiple buys.
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 ).
boardgames.stackexchange.com/users/1885/The Chaz 2.0
If you don't have all the expansions they're great for proxying other cards if you don't have attack cards that would use them.
I only have the base set right now, and I've used curses to represent Colonies, platinums, and even more complicated cards from the other sets if everyone understands them.
The blanks are also great for this.
You can use kingdom cards as well, but I find its easier to forget that they actually mean something else.
boardgames.stackexchange.com/users/191/Gordon Gustafson
Should  Curses  always be laid out as a supply pile even when no kingdom cards give curses? Or should the only be available when a curse kingdom card is chose?  An extension of this question could be should Curses be able to be purchased without being forced to get them by a card.
When should Curses be used?
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<dominion>
boardgames.stackexchange.com/questions/2093
boardgames.stackexchange.com/users/560/Pithlit
Curse cards are barely used in the basic game. You can buy them just like any card. The most common scenario is in a two-player game. Curse cards are present in every game, and you can buy them just like any other card, however, they are rarely used in the basic game other than with the Witch card.  When playing with "Fairgrounds" from the Cornucopia expansion you can buy a curse just to have another type of card in play.When playing with "Fairgrounds" from the Cornucopia expansion, for example, you can buy a curse just to have another type of card in play. The most common scenario for purchasing curses is in a two-player game where buying all ten of them would end the game, resulting in a win for the player who does so .
Curse cards are present in every game, however, they are rarely used in the basic game other than with the Witch card.   You can buy them just like any other card.   When playing with "Fairgrounds" from the Cornucopia expansion you can buy a curse just to have another type of card in play. The most common scenario for purchasing curses is in a two-player game where buying all ten of them would end the game, resulting in a win for the player who does so .