Straight from the eBay help pages:
Circumventing Fees Policy
It's important to remain in good standing in paying eBay fees. Sometimes sellers try to find ways around paying fees or do so by mistake. Please see "Some Examples" (below) to make sure you're following eBay policies.
Breaches of this policy may result in a range of actions, including:
Listing cancellation
Limits on account privileges
Account suspension
Forfeit of eBay fees on cancelled listings
Loss of PowerSeller status
Spend a few minutes taking eBay's tutorial on fee circumvention.
Some Examples
Outside-of-eBay offer - Listing an item on eBay and then being paid outside the eBay system through the eBay Web site.
Unreasonable postage or handling costs - Listings with low prices but unreasonably high postage or handling costs.