You are right. Values should be taken as they were at the time. You can't predict the future or what traders were thinking. Let's say a hat sold for 20 white paints a week ago. That would be 44 keys when the trade was made. White paints are going for a half a key in the store now. Trade shouldn't be valued at 10 keys. Same logic applies to all items. Values should as closely as possible represent what the item was worth at the time. Just note that this is the "real" value at the time of the sale. Bp.tf value at the time of the sale may not represent its "real" value. Consider unusuals that weren't updated and had a 5 month old price at the time of a sale. A newer suggestion made after the trade would be more likely to represent the value at the time of the sale.

Anyways, close enough here, it doesn't matter too much.