Right now, I'm level 31 and have almost 25 million roubles in the bank, about 15k dollars and 8k euros.
The primary way that I make my money in Tarkov is by doing Scav runs on Interchange at night. I rarely hang out inside the mall and just hit up the secret stashes scattered around the perimeter of the mall. There's numerous guides on Youtube and maps showing stash locations.
This guide is more geared toward what kind of loot you should keep, sell to traders or put on the Flea Market.
I highly recommend keeping https://eft-loot.com readily available while you play, just in case you wanna check the price of an item quickly.
As a general rule, anything people need to upgrade their hideouts are going to bring in some really good cash.
Things to Sell on the Flea Market
Any of the loot items in that image that don't have a little checkmark are good to sell on the flea market. Keep in mind that the ones with the checkmark you should keep for your own quests.
Keys are a excellent thing to sell on the flea market as some keys can go for millions of roubles. Check every jacket you possibly can find and grab every key. Use that eft-loot website and check the price of any keys you find to see if they're worth extracting with. Just earlier today I found a goshan cash register key worth 500k roubles on the flea market.
Sell Hideout Crafted items
Keep any Powercords you find and turn 2x Powercords into 8x Wires in the Hideout Workstation. I routinely sell a stacks of Wires on the flea market for a quick and easy 150k.
I also take all the PS 5.45x39 ammo I find off scavs and craft it into Blue Gunpowder, then take 4x Blue Gunpowder and 1x Green Gunpowder, trade them to Mechanic for an Ammo Box. Then sell that Ammo Box to Therapist for 120k.
Zibbos and Cricket lighters can craft a blue Fuel Tank, use the fuel for your hideout generator and then use 2x empty Blue Fuel Tanks, Metal Scissors, 3x Bolts and 3x Nuts to make a Magazine Box. Sell that Magazine Box to Therapist for 318k.
There's plenty more ways to make money with your hideout but these are my favorites.
Things to Sell to Traders
Guns, Armor, Backpacks, Chest Rigs, Ammo, and Health items are generally easy to come by so it's not really worth it to put that stuff on the flea market.
Just repair what you can and sell the Armor, Chest Rigs, Gun parts to Skier.
Sell the Guns to Mechanic.
Sell the Medical supplies to Therapist, or keep the meds for your PMC runs.