I am not sure why they have it worded that way in the rules. I think part of it was you could use the OEM seat in the early days. The basic rule of thumb I use when doing BS inspection is that if its needed to pass tech or its exempt, you can't sell it to reduce your cost You would basically be double dipping. You can sell your seats, because you have to replace them. You can sell your glass moon roof because you can't have it installed. You can't sell your windshield or wheels because you need those. You can sell off trim pieces, radios since they are not required to race/pass tech.
My experience is you don't get a lot of money for trim pieces unless its a rare car. It may help offset the costs but it isn't going to get your $3k+ car down to $500.
My standard caveat applies, bring something terrible and I don't care. If its British, French, Italian or more than 40 years old, no one cares just make it safe.
1992 Saturn SL2 (retired) - Elmo's Revenge - Class B winner, Heroic Fix winner x2
1969 Rover P6B 3500S(sold) - Super G-Rover - I.O.E Winner, Class C Winner
1996 Saturn SW2 - Elmo's Revenge (reborn!), Saturn SL1 Dazzleshipm Class C x2 and IOE winner
1974 AMC Javelin - Oscar's Trash heap - IOE,”Organizer's Choice" and "I got Screwed" award winner