To ensure that you treat all of your suppliers fairly and make objective business decisions, you should not accept free tickets or purchase tickets from your suppliers.
Accepting free tickets from your supplier would be considered a gift. If you buy the tickets at face value, you may not be paying the actual fair market value of the tickets. The difference in the face value and the fair market value would also be considered a gift.
Even if you paid fair market value for the tickets, you have access to the tickets because of your business relationship with your supplier; therefore, it would not be acceptable to buy the tickets from your supplier.