Have you seen some of the defending fleets around those pirate bases? They've clearly got plenty of credits and supplies.
More seriously, as for the "exploit" nature of it, I'm not sure I understand. As you sell stuff to the same place, the amount you receive in payment goes down, so if you sell too much, your return per unit goes way down. So even if you find a lucrative trade route, it'll go away naturally quickly. And its not like you can produce an infinite amount of loot without actually putting effort in.
More seriously, I don't see the benefits from a gameplay perspective. You've defeated the fleet and salvaged it for scrap or succeeded from coming from the deep black after particularly lucrative finds, and now you've got to spend a bunch more time just flying between planets to sell everything? That doesn't sound exactly fun to me.
From an immersion standpoint maybe, although just because the station itself is poor or in poor condition, doesn't mean there aren't rich well connected buyers there avoiding taxes - the black market by definition is not associated with those running the station. Also, the amount I sell tends to be peanuts compared to mission payouts or colony income. I feel like some of the larger delivery missions can have payouts in the hundreds of thousands of credits when delivering to small stations.