I sell $5 Amazon gift cards for around 7,500 credits all the time. Don't believe the people offering ridiculous low GIN offers. Watching and will be bidding :)
People use $1 per 1000 credits hun. This will give you a idea. So if you had $100 gift card it equals out to 100000 credits. Plus you need to add fees they charge you. I would look at what others do. If they have 100 gift card for 499 that usually means people are shill bidding to push it up. I hope this helps.
There's debate about credit values. I always go with 1000 credits = $1, because that's usually what it translates to for the Listia $50 amazon cards (around 50,000 credits). If you buy listia credits instead of earning them by listing your own stuff you pay a while lot more.