Thrifty Bill's badly stuck seat post discount = cap value at other parts on the bike (if there are other parts). Ultimately, you are talking cutting it out, as lye will ruin paint. Cutting it out is a PITA and if you are not careful, you can ruin the frame, ask me how I know.

Badly stuck parts = buyer takes all the risk. I've learned to cap my potential losses. If someone else is confident and willing to pay more, so be it. IMHO, the "benefit" from unsticking stuck parts should go to the person that gets them unstuck. If the seller wants more, I will make them an offer based on them getting the offending parts out.