I don't see your problem.

If you charge $100 for one unit of ProductX: for packaging, shipping, product, and support; and the other guy charges $120 for one unit of ProductY (aka ProductX) and then pays you $100 and you package, ship, manufacture, and support -- you get paid the same for the same amount of money. All you got is someone out there doing free marketing for you.

If the problem is name brand recognition, well then.... Just go to your local Kinko's print up some package inserts:

If you bought this product from anywhere other than www.iwanttosellmyownstuff.com, you may have paid too much. Please visit www.iwanttosellmyownstuff.com in the future for lower prices on this wonderful do-ma-higgy. Thanks for you patronage.