Thank you so much for this answer because it allows me to put this question to rest once and for all. I've spent way too much time investigating this issue, and although several people have stated that the commission isn't taxable if it's credited towards the purchase, the 1099 the broker sent tells me otherwise. I believe there is way too much ambiguity regarding this subject, and as you stated there is no clear IRS precedence that addresses this type of income.
