I have come across this, mainly with Coldwell Banker affiliated listing agents. It appears to be a company policy with them and is really no big deal. Your broker can easily provide you with a blanket W-9 (completed and signed by the broker) and you can copy and forward it with your offer. Remember, commissions are paid to the broker, not the agent, so the W-9 must contain the broker's information. Hope this helps.
Shel-lee Davis, CDPE, SFR, QSC
Your Real Estate Consultant for Life
RE/MAX Palos Verdes Realty
Their tax ID# needs to be given to the IRS (according to our accounting dept). Once we have their tax ID number on file, we dont require another form for future closings