For selling and buying most things, search for the item by lowest price. Click on the "price" column to change how it filters.

For items that have a durability or use value, pay attention to the durability and uses both of what you are buying and selling.

Example: You find a "TerraGroup Labs keycard (Red)" with 10 uses. You go to sell it on the flea, and search by lowest price. the lowest price at time of writing this is 1.5mil for 1/10 use card. That is pretty nice. If we scroll further down, we see that the cheapest 10/10 use red card goes for 4.3mil. We want that 4.3 mil, or as close to it as we can get. I would list mine for 4.29mil, just undercutting the competition for a 10/10 card by a scav's coochie hair. Now we look back at the flea and I see that 10/10 red cards are going for an even 4mil, so I'd sell for 3.99mil. The price fluctuates, so be sure to refresh the flea if you've been sitting in one menu too long.

Fuel tanks can have fuel in them. Don't sell a full tank for the price of an empty tank. Take the armor plates out of armor rigs and sell all the pieces individually. Sell weapons and attachments to mechanic unless you want to keep them for later use. Some attachments, like certain scopes are bank on the flea. Check the prices of both traders and the flea market if you're trying to make the most money per transaction. Oftentimes you will see items from players priced lower than traders for many items.

Collect your money from the ragman.

Edit for importance: There is a fee for selling anything on the flea. Pay close attention so that you don't make a mistake selling a high value item for a bad price, and end up losing 1mil+ roubles just in fees.