Submitted by pedroais, also found by leastwood and WatchPug
A primary seller can circumvent the 15% fee and pay 5% as a secondary seller.
The Foundation protocol charges a 15% fee if the sale is a primary sale and 5% if its a secondary sale.
https://github.com/code-423n4/2022-02-foundation/blob/4d8c8931baffae31c7506872bf1100e1598f2754/contracts/mixins/NFTMarketFees.sol#L40
There are 2 conditions that must be met for a sale to be considered primary:
https://github.com/code-423n4/2022-02-foundation/blob/4d8c8931baffae31c7506872bf1100e1598f2754/contracts/mixins/NFTMarketFees.sol#L188
Both of these conditions can be easily circumvented by the primary seller.
With any of these 2 methods, the primary seller can circumvent the 15% fee and pay 5% as a secondary seller which makes the primary seller fee optional to pay.
NickCuso (Foundation) acknowledged amd commented:
