So just to be clear the seller can always have access to the address? So a buyer should not use it to store coins and the seller can maybe take these coins?
Yes. You can create a signed message with the old address stating that you bought the account and want a new address to be associated with the account. Post that signed message in the stake thread with the bought account to let someone else quote it. This essentially transfers ownership to a new address over time. Newly staked addresses do not matter much, but they matter more the older they get without incident.
Also electrum 2.5.4 does not have an import button.... only a sweep button? how can you import the private key then?
You just explained it above. Restore, use the private key (not(!) the address) as the seed. It will create a new wallet file only for this key and address.
Thanks
Edit links:
Stake thread -> https://bitcointalksearch.org/topic/stake-your-bitcoin-address-here-996318
Learn to sign a message with electrum -> https://bitcointalksearch.org/topic/how-to-sign-a-message-990345
ok thanks