You can use Coins gained in the Main Phase to buy cards from the Market. Each card has a specific cost in Coins that you must pay. Coins that you spend are of course lost, but so long as you have enough Coins left, you can buy as many cards as you wish. Newly bought cards go from the Market directly to your discard pile.
Example: You currently have 5 Coins. You spend 3 Coins to buy a City card, and 2 Coins to buy a Contribution card. The newly bought City and Contribution cards go from the Market to your discard pile.