A wallet can change hands without moving, private keys are being traded, leave em alone!
Who would accept a private key from someone knowing that the original ownwer could have kept a copy. I understand the concept between family members etc but besides that I don't see any advantages.
In reality you always have to trust the party you buy a Bitcoin from if you want anonymity. If the guy you buy them from earned them on SR, they will come kicking your door in too, when they are looking into what the seller spend his coins on!
An untouched wallet or cascasious, where only mined coins are put in, is totally disassociated from any identity. In reality, if you spend it, it is no longer anonymous as if the guy you pay it to has his wallet linked to a bank account, someone can ask him where he got it from.
Thats the price of mainstream!