I would assume there would be automatic conversion. That's far more common than making the player keep track of all their copper/silver/gold/platinum/electrum/steel pence/harbor moons/whatever. Larian isn't going for a Simulationist sort of game where you have to keep track of travel hours and set watches or pack animals etc (a fun as I might find a game like that) I don't think that sort of extra work for the player would be appropriate in a game like BGIII. But decompressing the values of lower-end items would make looking for treasure through the piles of junk a bit more enjoyable.
Aside from the odd piece of silverware or gems there really isn't much to be found amongst all the rubbish to make me say 'oh yay'
I'd also echo the suggestion of specialist merchants. I often instinctually horde stuff like books or seemingly-rare commodities and luxuries because it makes sense to me that (for example) 'somewhere I'm sure to find a buyer in BG for this collection of rare books/fancy wines etc'.
I really don't like the attitude + bartering system. I'd like it if every merchant just had different price settings for goods. Like 'doesn't buy X', 'Buys X for reduced price', 'Buys X at normal price', and 'buys x at increased price'. So maybe a jeweler could buy your rubies for more than the general store, but won't buy your pot lids and goblin scimitars at all etc.
Some other suggestions for treasure that Larian could use to spice up looting-luxury trade goods like expensive textiles or spices, & trade bars.
