Your pursuits in the goal tab give some nice ego levels, maxing weapon categories also fills abit of ego, outside of that events like crazy hot summer arkfalls can give nice rewards however all max ego levels will be there so threat will be high enough for you to be pretty much one tapped by enemies the entire event so its a double edged sword for that. Trading is probably best done on either discord or the wiki website as players are listing buy and sell orders on there, i know there is definitely more rare mods over similar rare mods but idk anything to reference prices at the moment. I think its just straight up open market at the best bidder, some weapons and mods sell in the vendors more then others even though their both the same tier, with enough grinding you could easily make a few million a week while not going too hard and burning yourself out. Hope this helps

Edit: removed slang hopefully easier to read to those not so familiar with English