I've seen it said several times by several people in many forums, here and elsewhere, that the OpenStore Markup/Markdown values set the Markup/Markdown percentage when the store is opened.
The Markup/Markdown are actually bonuses
; added to the values that are already set in the UTM file.
Thus, a store has a 125% markup and a 25% markdown, the command:
results in buying and selling at 0%; all items bought and sold at 0 cost (as adding a negative is subtracting).
causes the store to sell at a whopping 250% and buy for zero; and the command
causes the store to sell at base value and buy at the 25% so stipulated in the UTM file.
ALSO, it appears that both numbers run the same direction: Up! Thus, 25% markdown is misleading; as it's not "marked down" from 100%, but rather, sells at 25% of the 100% base value.