so you use multibit, but you dont know how to sign messages??
that smells fishy, have you ever checked your multibit tools options?
signing msgs with multibit is close to beeing the easiest thing in the world
If i use multibit that doesnt mean i must know how to sign messages.
If you say it is eassy then i see no problem verifying account is mine.
This will be done when the buyer appears, to not waste a time