Why not change perspective. Make the PIF only items under 20 bucks. (5 dollar min value-20 max value) keep all the other rules and no one have expectations of value. Really just pay the free gifts forward . If someone puts a 20 dollar item up and the next guy puts up a 5 dollar item , who cares? If you ever wanted to add a higher value tier, you could make it that only guys with successful offers get to participate. The giving part is the fun, it will never work with high valued item's, trades are also not popular. Add the rule that anyone crying over value is out!
Higher value offer...xxxxxxxxxx...... Only members with 3 successful offers can claim this !(random offers)