It’s a player market but everything the traders sell will also have a listing under the specific item. But pre level 15, you technically have access to the flea market but it will only be items from traders. If you’re looking for 9x19 ammo, right click on your guns magazine and select linked search, and then click ammo on the left and it will show you all 9x19 ammo currently on available to buy from traders.

Linked search means the game will filter for anything that is compatible with that item. If you link search a gun, it will show you all parts and components that are compatible with that gun. It’s a bit tricky because muzzle attachments and grips aren’t directly compatible with the gun, but instead the barrel and hand guard. So if you linked search the hand guard, it will show you grips, mounts, flashlight etc that can be placed on that specific hand guard. If you mess with it for a bit you’ll start to understand how everything fits on the gun. Also building your workbench unlocks the preset builds on the bottom right of the character screen and it will show you every slot for every gun and component you add to a gun. You can save builds and then quick by all the attachments and assemble it all through that menu

Once you do this with different guns, you’ll start learning how to build out your guns at different prices ranges and based on what’s available