Just immediately to the right of your character panel (where all the items are), you'll see several tabs. Usually the highlighted/selected one is the tab with the infinite sign (which shows all the items except keys and equipped items). Below this, you'll find an "equipped items tab." This shows the character's (the one you're using to barter and not necessarily the one that initiated dialogue) equipped items. So far, this is the best way to compare items from what I know.
As for the gold, no matter who's holding it, the game automatically detects the sum amount. Just use the "balancing scale" icon above the bartering panels. You'll see what I mean.