In the past day I have already managed to buy 4 Haunted Souls on Outpost for 1 scrap each.

http://www.tf2outpost.com/trade/6100036

The market is saturated so no ones wants these.

Seriously, it's better to ust keep then and save then for the next halloween. This year we had a low number of gift-farm servers using koth_viaduct_event and cp_manor_event and the halloween costumes and masks went up in price, so next year the same thing will happen with the cotumes and the souls.

tl;dr: DO NOT sell your souls. Keep them for the 2013 halloween.