Hello and welcome to the LE community!
There will be cycles, but the first few ones will not have content difference between them and legacy.
There is a distinction between trading and gifting.
Trading is done via the merchant guild, which is one of the two faction choices.
You should read about that here:
Trading will be available through the bazaar and direct player to player trading but with a lot of restrictions.
Then there is the gifting which can be done with player that you played together with when a item dropped or if you played logn enough together there is a ressources called resonances that you cna use to gift items that the other player was not present, when it dropped.
Up to 4 player parties are possible.
The mastery is the only thing that is not respeccable in LE, everything else is respeccable with eease.
The mastery is more a class choice than anything else. This is the most important decision you have to make, but within each mastery there are plethora of buidls and options available.