+Buy
Playing a card that provides +Buy increases the number of cards you are allowed to buy during your buy phase.
+Buy is an essential component of most engine strategies, since engines can take a long time to build; +Buy allows you to acquire engine components more rapidly, thus speeding up the construction of the engine, and once the engine is running it allows you to buy multiple Victory cards at once to make up for the slower start. An easy trap to fall into is that of "overbuilding" an engine in the absence of +Buy: there's no use in building a deck that can regularly generate 
each turn if you can still only buy one Province with it. In some engines, it is good strategy to wait to buy even a relatively cheap +Buy card until the engine is regularly producing lots of 
each turn, though, if the +Buy card doesn't itself help the engine get moving—"paying 
for Herbalist".
In a Big Money strategy, +Buy is usually less useful, since on most turns in most Big Money decks you just want to buy the most expensive card you can, and you'll rarely have a turn with enough to buy both a Province and something else useful. The opportunity cost of getting a card just for its +Buy is usually too great in these decks.
Alt-VP rushes also usually benefit from +Buy, since these decks depend on gaining large numbers of relatively cheap cards, and extra buys can often be used even on Copper rather than go to waste.
Gainers and remodelers are sometimes referred to as "virtual +Buy", since they provide other ways to gain multiple cards per turn. Gainers and remodelers don't depend on having enough money to spend to buy additional cards as +Buy cards do, but they are usually less flexible in other ways.
Some cards that give +Buy have it as part of their core function, such as Nomad Camp; usually the +Buy is the only reason for wanting a Nomad Camp. In some cases, a card offers +Buy as a feature that synergizes with its core function: Goons offers you a + 
reward per card you buy, and gives +Buy to make the reward go farther. Other cards have +Buy merely as an extra vanilla bonus that keeps the card competitive with other cards at the same cost: Margrave is a useful card even in Big Money if you never use its +Buy, but the fact that it has +Buy compensates for the fact that its attack is not very strong and makes it easier to include in an engine.
Cards that give +Buy
Cards in italics have been removed.
Terminal Coin/Cost reduction
- Dominion: Woodcutter
- Intrigue: Baron, Bridge
- Seaside: Salvager
- Alchemy: Herbalist
- Prosperity: Trade Route, Goons
- Cornucopia: Princess, Horse Traders
- Hinterlands: Nomad Camp, Nomads, Spice Merchant, Souk
- Dark Ages: Squire, Storeroom
- Guilds: Merchant Guild
- Adventures: Peasant, Messenger, Bridge Troll, Wine Merchant
- Empires: Farmers' Market
- Nocturne: Sacred Grove
- Menagerie: Animal Fair, Way of the Monkey
- Allies: Herb Gatherer, Town, Galleria
Black Market does not technically provide +Buy, but it lets you buy an additional card during the Action phase.
Terminal draw
Cantrip
- Market
- Worker's Village
- City (but only after 2 Supply piles are empty)
- Grand Market
- Hamlet
- Market Square
- Snowy Village
- Sanctuary
Treasures and disappearing money
- Dominion: Festival
- Seaside: Astrolabe
- Prosperity: Tiara, Collection, Contraband
- Dark Ages: Forager, Counterfeit
- Guilds: Candlestick Maker
- Empires: Villa, Capital, Charm, Fortune
- Nocturne: Pouch
- Renaissance: Ducat, Spices
- Menagerie: Stockpile
- Allies: Bauble
Other
- Pawn can provide +Buy in combination with either +Action, +Card, or +
. - Courtier offers +Buy as one of several choices.
- Tactician gives +Buy on the next turn only.
- Ruined Market provides only +Buy; it's occasionally claimed that this is the one Ruins card you might intentionally gain in order to build an engine.
- Sir Martin has +2 buys as his vanilla bonus in addition to the Knight attack.
- Travelling Fair is a sort of "+Buy Village", turning 1 Buy into 2, while also letting you topdeck cards you gain.
- Seaway and Teacher can give +Buy to an Action card pile in the Supply.
- Forum gives +Buy when bought.
- Druid provides +Buy plus a choice of one of three Boons.
- Skulk provides +Buy and a Hexing attack.
- The Forest's Gift provides +Buy and + as a Boon.
- Fair provides +1 Buy every turn
- Scrap offers +Buy as one of several choices.
- Band of Nomads allows you to spend Favors for +Buy.
- League of Shopkeepers can give you +buy when you play a Liaison.
Synergies
- Engine strategies in general
- Alt-VP rushes and deck-bloating strategies such as Gardens, Silk Road, Vineyard, and Philosopher's Stone
- Non-terminal cost reducers (e.g., Highway and Quarry)
- Goons
- Peddler
- Potion
