The 256x item selling price glitch is a glitch in Pokémon Ruby and Sapphire.
Under normal circumstances, it only occurs with Berries, the only items possible to have over 99 of without using a cheating device or glitches.
When selling over 256 of an item, although the correct sell price is displayed (both before and after selling), the actual amount of money received does not match it. This amount is calculated not with the actual number of items sold, but with this number modulo 256 (i.e. the lower byte). Effectively, the total cost for every 256 of the item is ignored.
For example, Berries cost 10 Pokédollars each. If 258 were sold, the total amount of money gained in this case would be 10 * 2 Pokédollars, or 20 Pokédollars (a loss of 2560 Pokédollars), as the group of 256 are not counted. In another example, if 999 were sold, the total amount of money gained would be (999 modulo 256) * 10 or 2310 Pokédollars.