6.1.15. Do not infringe on globally-recognized trademarks by listing items for sale that violate the trademark holder's rights. Selling intellectual property or benefiting from the intellectual property of another party's without their permission is strictly prohibited.
This includes typographical errors of globally-recognized trademarks ("typos").
You have to tread carefully with trademarks, but there are plenty of uses for the letters f & b.
I'm not keen on the extension, though some .xyz have fetched decent money. This being a two letter might be one of those. If I were you I'd probably just hold it until I knew what to do with it