B/Os alone can only be used to drop the value on a hat if all three of these criteria are met

​​You are dropping the value on the hat (you cannot raise the value on a hat based on a b/o

You must drop the mean value of the hat by at least 15%. (i.e. You shouldn't be dropping a hat from 31 to 30 keys based on a b/o. You SHOULD be dropping the value on a hat from 100 to 50 if 50 can't sell in a month).

The trade must have a reasonable time to mature (and must be reasonably bumped). What is reasonable varies from hat to hat. A good rule of thumb is to go by roughly 25 keys per week. So a trade for a hat <25 keys in value must be up for at least a week. A trade for a hat <50 keys must be up for at least 2 weeks. A trade for a hat <75 keys must be up for at least 3 weeks. For all other hats, the trade must be up for at least a month in order to price them based off the b/o.

http://forums.backpack.tf/index.php?/topic/6033-guide-for-unusual-price-suggestions/