[Bampop]

It means that even if you sell Items worth 0, 100, or 200 Gold Pieces, you treat them as if they are worth 300. So you could sell 4 Items with No Value printed on them and still get to go up a level.

The part about modifiers relates to other Race or Class abilities such as Halfling who let's you double the value of one Item, or some other Dungeon you may be in, or maybe a Curse! that's affecting you.