Yes you can trade offline, ever played Runescape? They have this thing called the grand exchange were players can either purchase or sell items. If you want to purchase an item you have to set which price you want it for, if another player sells this item for the same price it’s a match and both of you trade.
So if we use a system similar to this, trading while offline will be possible, and because the grand exchange doesn’t care who the player is, you can’t really sell all of your items from other accounts and make sure they safely get to one account without another player buying them(especially if it’s a legend only item).
Example of this system.
You pick an item, then you choose either ‘Sell’ or ‘Purchase’ after you made your choice you will be able to set the price of it (in gold).
Done! Trading offline is possible and you can’t abuse this sytem like other trading systems because before you switch accounts another player has probably already purchased it if it’s in a really low price.
Just gotta learn from runescape bois.