I haven't been playing this game very long, but one thing I've noticed is that people aren't taking advantage of barters from traders, even though they can save you a ton of money in the long run.
Before buying something off the flea market, check to see if a trader is willing to barter for that item. It is often times cheaper to buy the barter items off the flea market, and then barter with the trader.
Let's use an example. A CMS kit on the flea market goes for roughly 29000 roubles right now. However, you can barter a CMS kit from therapist for two bug repellents. A single bug repellent costs roughly 8000 roubles right now. 8000 x 2 = 16000. If you buy two repellents and barter for the CMS, you'll save about 13000 roubles.
Let's use one more example. The MP-153 is certainly one of the more popular shotguns in the game, and sells for about 29000 roubles from Jaeger (LL2). However, there is a barter for an MP-153 from Skier LL1 that requires a car key (3000 roubles flea market price), a Yotota key (3000 roubles), and a VAZ key (4500 roubles, also known as a sixpack key). 3000 + 3000 + 4500 = 10500. If you do the barter from Skier, you can save about 18500 roubles.
All of this is assuming that you buy the items necessary for the barter off of the flea market. If you are rich and don't really need to do any penny-pinching, then this really isn't helpful to you, however, in the early stages of a wipe, this can be a lifesaver.