Rux will sell whatever legendary or super rare items he finds. He might have a higher chance of finding some legendary or super rare pieces if you have most of the common/uncommon items. This might also work for the vending machines because I have brought all the characters (except for the legendaries) and whenever I buy something from the vending machine, it usually gives out some random common - super rare items depending on the number of stickers you have collected.
If Rux has what we call a 'Legendary' visit, then he usally does not offer us an ability, at least out of the two legendary visits we have had. I personally am hoping for the exclusive abilities, as they can only be gotten through Rux, unlike legendary stickers, that can be aquired through packs in the normal shop, if even with a low drop rate. As a side note, I find that I recieve a Legendary Sticker about every one in fifty packs.
With Dark Bean confirmed, I doubt there will also be a character piece offered. But who knows? He'd just better not offer the bugged Dillo Derby hat for engineer.
