How do sites like PlasticJungle verify gift card balances and prevent fraud

I have the below 2 questions that I was hoping somebody from the community would probably be able to help me with:

1. Say I sell a gift card to PlasticJungle (or other similar sites). How do they verify the balance on the gift card? Do they check it once they get the physical card (I send them via mail) or do they have some sort of APIs exposed by all major retailers to check the balance electronically when I key in the gift card number and the pin on their website? Though the latter doesn't sound like how it's done, thought would still ask.

2. How do they prevent fraud? Say I mailed them the gift card. After getting it, they check and confirm that everything looks good and then they proceed to pay me (check, paypal, amazon gift card). Now once I get paid, what if I go ahead and use the gift card online since I know the gift card number and the pin. How do they prevent such a thing?