Can't be sold item

What it's actually checking for there with that BEQ at B63F is if it's a null item -- FFh -- aka an empty slot. You're absolutely right that /B708 is (probably) too late in the process; I'm trying to find where it loads item information so I can find where the item price is stored.

Looking at it, the code doesn't actually load the item price until the start of the function at /B708, with a jump to C3/BB65, which makes it difficult to check against it; I haven't tracked down where it actually draws the sell menu yet, which is where we'd need to look at greying out items.