These notifications could be:
"You got outbid on (Card) by (who did it)"
I feel like this is the one that would be the most useful as you don't have to consistently check on the card to see if you've been outbid.
"Someone bid (amount) on your card (card name)"
This one can help you in a number of situations via when someone underbids on a card you're trying to sell and you don't check it.
These are only two examples because I need to go the bed but I feel like there could be a lot more added.
Also remember I suggest that they be toggleable so that people don't get spammed
thoughts?