In trading, B/O refers to Buyout, or how much currency players want for their
specific item they are trying to sell. Depending on the stats, the B/O can
either be very low, from 1,000g, or very high, such as 600,000g. You can
always talk the trader into trying to lower the price to make it easier for
you to buy.

EXAMPLE: Selling a Gold Unid Storm Bow (B/O - 100k).