I've only been playing for a couple of months but have been noticing a ton of listings on the Flea Market where the seller is "losing" out on some money.
For a lot of items that aren't used for hideout upgrades (keys, weapon parts, bitcoins, injectors, etc.) you'll actually earn more money by selling to Therapist/other dealers than you'll get on the Flea Market. This is especially true when you take the listing fee into consideration.
Easy example is bitcoins –
- Flea Market: 150,000 sale price – 9,000 listing fee = 141,000 profit
- Therapist: 149,434 profit
Often you can even buy bitcoins on the flea market for under 149k which you then turn around and sell to Therapist for a profit. The prices do fluctuate a bit so check often!
This holds true for many other items so reminder to check Dealers!