I am an online retailer with online stores on two popular internet sets. I have an anti-compete issue

It may or may not be legal, depending on the particular circumstances.  It could be illegal if it violates antitrust laws (which is a very fact-sensitive issue) or if you are being discriminated against due to a reason prohibited by statute (e.g. race, nationality, religion, etc.)  However, it is not illegal per se for a distributor to refuse to sell to some outlets.  For example, it is common for a manufacturer to have a contract with a distributor that the distributor will not sell to off-price retailers, or sell to those it knows will sell the goods outside of the distributor's territory, etc.  If McDonald's wants an agreement with Pepsi to sell Pepsi beverages at McDonald's restaurants, you can be sue that Pepsi will insist that McDonald's sells no Coca Cola beverages.