Every time you increase the level of your battle exp., Infrastructure, or Raiding (flight exp), you get 1 crystal. The fee for selling stuff depends on how much you list; but to give you an idea, I can list 50 battleships and pay a fee of only 2 crystals if I put the price in the recommended range; I can sell 10,000,000 metal and pay a fee of 3 crystals; I can sell 10,000,000 minerals for a fee of 7 crystals.

As you can see, the sale price is vasty greater than the listing fee; just remember that if a lot sits unpurchased for 30 days it will be returned, but the fee will not be refunded.
Also, Terran accounts access the bestiary via the PSI Disruptor, but the Disruptor can be destroyed in battle regardless of level.

Zergs, build a level 4 neural core, then build the building known as the bestiary. Once you've done that, go to the trade screen xcraft.net/market, scroll down to bestiary, and click the button that says establish a connection. After that, click the exchange resources tab, type in what you want to sell and what you want in return for it, then hit the button on the bottom of the screen. A gigalord will come to collect the resources, and they will be listed for sale once it arrives at your planet. Please note that there is a fee in hydarian crystals for both listing and purchasing a lot - in this case you pay the listing fee and whoever buys the lot will pay the purchase fee, but if it sits for 30 days without getting bought it will be returned to the planet it was picked up from, but the fee paid to list it will not be returned with it.

how does that work?

Robie7979, If your fleet on our terran account is vastly larger, that would explain it. The event also halves your amortization costs, as well as halving the time needed to research technologies/evolutions/disciplines
For me the event costs 33 crystals, which is about equal to the value of my production and amortization added together
On my zerg account the cost is 19 crystals, but my fleet on my zerg account is much smaller - although the overall production on all my planets is slightly higher.

Can you give an example please