Depends who we’re afraid of here. Are we afraid of whoever lost the suitcase full of money, or the government, or both?

I’d assume whoever ditched a suitcase of money is a criminal, and unlikely to report it as lost with the serial numbers of the banknotes.

In that case I’d just use it for some day-to-day stuff, small luxuries, and whatever crap I want on Facebook marketplace that doesn’t need to be registered.

If I was convinced these banknotes came from a bank robbery or something of the sort, I think I’d only use it at somewhat sketchy looking flea markets / swap meets, and on holiday in foreign countries. But in that case it would be very hard to ever use all of it without trying to get a suspicious amount of cash through TSA and past customs.