Haggling Items

My personal rules on selling items:

The PH says items that are sold are done so at 1/5 the market value. Players can haggle with a vendor (3 successes before 2 failures at a DC 20 diplomacy check) to sell an item for 1/4 its price.

If successful, Players can further sell items for 1/3 the price with 3 successes before 2 failures at DC 25.

After this, attempts can be made to sell at 1/2 the market value with 4 successes before 2 failures at DC 30.

For the sake of time, this can only be attempted ONCE for each item and only with magic items. If players can find a vendor of the same race that is interested in the item, they will receive a +1 bonus to all checks.