Ethically speaking - you probably shouldn't do them if you're a REALTOR.
Like Ben stated - sort of - If you're not an expert in the area you would be providing the bpo for - then that would be unethical.
Of course there are other MLS rules to be concerned about, which would be fines and possibly expulsion for your client as as well as licensing laws in the states you would be completing that you are not licensed in - and that would fall squarely on your shoulders - but it's good you are concerned about the ethics portion too - it's a beginning

I wonder how E&O would look at this, for all participants, lol