http://forums.backpack.tf/index.php?/topic/6033-new-unusual-price-suggestion-guide-the-cheat-sheet/

The values of items in a trade should be the "value" of the item at the time of the trade.

This can be very tricky as it can be difficult to extrapolate the value of an item on March 1 when the two most recent suggestions were on February 1 and April 1. In most scenarios, the value should based on the accepted suggestion closest in time to the date of the trade. For unusuals, this is usually the most recent suggestion for a hat since they are rarely updated, but this is not always the case. Many new unusuals are updated every 2-4 weeks.

Examples:

- If the sale happened on April 1 and there are two suggestions at March 1 and April 2, the April 2 suggestion value should be used.

- If the sale happened on April 1 and there are two suggestions at March 30 and April 30, the March 30 suggestion value should be used.

To further complicate matters, some items (like buds or the bmoc) drop precipitously. So using this method may not be reliable. To the best of your ability, try to use the "value" of the item at the time of the sale. If that is too hard to predict, that proof should be avoided in making the suggestion.