The best thing I've gotten from a scratch card is 10,000 NP which was from the most common kind (can't remember the name, having a major mind block >.< ). I usually sell any other scratch card I happen to get...I find that it's more profitable to sell them, like you said.
What type of scratchcard are you talking about? I only buy lost desert scratch cards. I've gotten the Sandtravaganza card twice, which i've sold for a good profit! I forget how much though. Scratching cards, most i've gotten is a scarab ring, so i don't bother. But there must be some profit to it, if people keep buying them. I guess the jackpot is tempting.
Race to Riches! Gah, I don't know why I couldn't think of the name of it!! It's late, I'm tired, maybe that's it, lol. I get all my scratchcards from the Ice Caves Kiosk. Maybe I'll try buying them from other places...maybe then I'll get more rare ones.
10K i usually only play the ice caves scratchcards. they seem to be the only ones worthwhile, to me anyway.
i only use terror trove scartchcards to try and level up but i seem to make a nice profit with them getting 10k and 5k quite often.
I think either 10k or 20k. I can't remember if the 20k was from Buried Treasure or a Scratchcard. Either way, I usually scratch them if they're like the base level card-- otherwise I sell them.