RE: trading private keys with somebody: theoretically you could, but that's not typical bitcoin usage and the current bitcoin client makes it hard to do.
Practically speaking, if Bitcoin did have a feature to state:
"On THIS date somebody using the identifier <foo@bar.com> controlled THIS public/private keypair"
... that would just inspire the smarter bad guys to send stolen coins to new private keys before spending them.
I can imagine a black market forming where dumber bad guys sell stolen wallet.dat files (at a discount) to smarter bad guys (or just plain greedy people who don't bother to ask where the wallet came from), who mix up the coins in them and then sell them.