My apologies as I completely forgot to add the description after posting it. I was trying to hurry and get it posted so it wouldn't end too late. I added an update that I believe answers your question. Thank you for asking!
GIN stands for Get It Now and is credit amount a seller can offer once they have been awarded their "Seller's Wings". If an auction had a GIN set at 15,000 it would be listed just beneath the Current or Starting Bid. This means that at any time, any person with enough credits can click on the GIN and win the item for that amount of credits. Having said that, Listia charges the seller 50 credits to list a GIN, which is returned if someone clicks to GIN. However, Listia takes 10% of the GIN amount, so if the amount was 15,000 credits the seller would receive 13,500 credits and not 15,000. Hope this helps =)