Just an observation from the last couple of sales I've made. People have asked for it and I've sort of danced around the question since I think there could be a sinister motive.
It can't be to pinpoint the year of manufacture, because on both occasions it's been stated quite clearly.
I'm not entirely sure what the negative connotations of someone obtaining the serial number are, I just know that there must be some, otherwise people wouldn't blur/paint them over when putting up pictures.
Perhaps they can report that the guitar was stolen and give your serial as 'evidence'? I dunno, help me out here.
Maybe to check if it has been reported stolen? I've always thought it was kind of silly to blur out the number. Just seems paranoid to me.