I've found a few scratchcards in the pools, and have no idea what to do with them other than sell them. Is there a "currency" option for them or something?

If you have them in your inventory, you can go to Games > Chance > Scratchcards in order to select the one you want to scratch. You can scratch 6 of 9 fields in a chance to win sP, a coin, a random item, stats, or nothing at all. :) Also, if it's the Golden Glory Scratchcard that you found, it's worth around 8mil sP, so you have to decide if it's worth scratching or if you better sell it.

Thank you!