You can purchase everything under the Equipment Tab of the Allowed Content from NPCs (some conditions may apply - please read the item details).
Items not listed under the Equipment Tab (e.g. magical items) can only be purchased from other players or recieved as loot from adventures.
Equipment purchases can be made during downtime, or in-session as allowed by the DM (e.g. when the party is at a market or in the vicinity of a merchant).
1 - Declare the transaction in #downtime-logs.
mydiscord_id as character-name (level) spends xx GP to purchase goods
Example:
@player1 as Robin Hood (5) spends 35GP to purchase 1x Shortbow and 1x Leather Armor
2 - Make an entry in your MAL:

3 - In the inventory section of your MAL, add your equipment:

If making a purchase in-session, you would log that purchase as part of your final session log entry. Add the purchased item(s) with your items gains, and subtract the cost of your purchase from your gold rewards.
Reselling equipment to the general market (to NPCs) is at 50% value of the original purchase price.
You can trade items with other PCs. Only PCs in the same location can trade with each other and any items you obtain via trading with another PC must be of your PC’s tier or lower. All trades must be logged in #trade-logs and both players’ MALs must be updated with the corresponding entries.

You can list your items for trade in #trade-listing. But please observe the rules for posts in this channel.
Note: You can’t trade gold or equipment that can be sold for gold between your PCs, even by proxy or cross character trade (such as trading an item from one of your PCs and receiving an item on another PC).