Well right now I believe you can buy 9,000 credits for $5.00 from Listia. Gift cards and items each have their own values. A gift card depending on the store is worth 2-5 times the value. eBay is probably the most expensive card to buy. Mostly because they are harder to find in stores.
It used to be that 1,000 was equal to $1.00 but now it's all messed up. :/ sometimes you get way less than your item is worth - sometimes you get more. It just depends. I paid $30 for a game boy thing and sold it for 33,000 which is like $15. :/ it's all wacked.
I bet it would be too complicated for Listia to fix which might lead to less fun for Listians. We'll have to stick with the honor system and hope most of us are honorable. The last time I sold something I knew was worth a bit I just put the credit equivalent as my starting bid. That way I was sure to get back my investment as well as shipping costs.