If you buy a bunch of resources in rapid succession, the price will rise.  If you sell in rapid succession, the price will fall.  The mark-up on the black market is absolutely massive, so in practice there is no way to abuse this.  Even at the low-point of the market you'll pay 400 credits to buy 100 metal or crystal, whereas at the high point you'll only get about 250 credits for selling 100 metal or crystal.  Bottom line is that even if you buy low and sell high you're still turning a loss.

Use the black market to correct resource imbalances you may have.  Don't try to play around and try to make a profit; the prices are fixed and you always lose.  You can try waiting for favourable prices, but usually I find it's not worth it.

If players are buying frequently enough for you to do this, the price will virtually never be low.  Selling to other players is a great strategy, but I don't think the margins are good enough to merit buying from the black market to sell to players.

I haven’t tested it but you might be able to make money on the black market if:

1.       Player one is TEC with favored client discount

2.       Player two does not have favored client discount

3.       Both players have 0% upkeep

4.       Player one buys metal or crystal

5.       Player one gives metal or crystal to player two

6.       Player two offers metal or crystal on the black market for 100% price

7.       Repeat steps 4-6

This is of course without the variable of changing prices, and I don’t think it’s very practical in a real game.