Signing the message from a Bitcoin address posted 5-6 month gives the buyer a idea that it was not claimed by anyone in last 5-6 months which means it a legal account not a hacked one and signing message also gives the buyer a kinda protection that he will not claim the account in near future and signed message will stop him to claim the sold account that's all why people ask for a signed message like this that the account belongs to me and I'm selling it for -- BTC.