Default tax in all fiefs is 5%.
You can increase/decrease that for 5PP per 1%, when you own that fief.
When you sell in a fief, the money you "produce" is reduced by the tax of the fief you bough from (buyer tax) and the fief you just sold in (seller tax) automatically. To have more benefit it is better to seek low-tax fiefs but you dont always have that choice.
35% seller tax seems like an overly-taxed fief, dont give them that much money and go somewhere else