as someone who has played MMORPG's for YEARS and someone who has been in the MMORPG development scene for just over 6 years, i can attest that this will be a double edged sword
some items that should not be trade-able
- Legendary's should never be trade-able unless another legendary is able to be traded, this is how that should work
- Ammo, Potions, cheat cards
some items that should be very limited to trading
- Ship Materials, jewelry and collection treasures
now obviously your going to be asking this what about gold??
welllllllllllllll heres where that gets complicated, gold would be the universal standard for trading, how this is done the right way is by first getting rid of the standard system all together, this means gold is now useless, to some players already their gold has hit that fine point of where its already useless to them as they are completely maxed out
restructuring how gold works is very very very difficult and challenging, there are several complicated ways of doing this however, what you would need to do is first put in a "worth" calculator with trading, this way items that are trade-able always have a "worth" which you can then tell the game what to do with it, what loot pools, what item stage brackets its placed in, etc, this also allows you to do some wacky things with them.
the second most important part to this is by making the gold system more refined by re-doing how you obtain gold, selling items to npc's, looting from bosses, pvp ships, etc is the current method, there is so much gold floating around the server that gold is practically useless in its own right, currently the cap of gold is 300k, that amount of gold doesn't really go very far once your capped out, maxed out, and have nothing else to do, ships as far as i know have 6 tiers of upgrading, clothes cost gold, etc, theres not much you can do with it, this is why making the gold system more refined by increasing the cap from 300k to say 1m and then adding content as you go for things that would be worth that much gold, would be very ideal for the economy as a whole, i mean really the sky's the limit.
i could go on and on about how to re-do the gold standard system, but that would honestly take up half this forum post alone, when it comes down to it, you can do alot of stuff