The general rule for setting a resale price is,
1. Find out how many years the seat has for a "full life".
2. Figure out how many years of life are left (so you have to subtract from the full lifespan).
3. Find the current selling price.
4. Divide the current selling price by however many years it has (for a full life). This gives you a rough price per month.
5. Take however many years are left and multiply that by the number you got for step 4.
6. Reduce for snags/tears/stains/missing parts/any "updated" model that is out.
Example: The seat has a 6 year life span, and it's current selling price is $300.
So you divide $300 by 6 (for it's life span). You get $50.
There are four years left, so you take 4 times $50. You get $200.
Then you knock the price down as needed.
As far as swap, here are the requirements to gain access to post things in Swap and how you can gain access:
Senior Community Members, Technical Professionals and any member with a
paid subscription may submit new threads (wanted, for sale, for trade, price checks) in the Carseat Swap (Carseats and CPS related) and Swap-O-Rama (all other items) sub-forums. If you do not wish to subscribe, free access is given to Child Passenger Safety Technicians who have requested access and to Senior Community Members (those Community Members who have been registered for 1 year, have 300 posts and 150 reputation points). No exceptions will be made. Anything resembling a for sale, for trade, in search of or price check post outside of the swap forums will be edited or removed at the discretion of the moderators.
(From Here.)
This thread
HERE also has details about access and each subscription level if you scroll down.
So for you personally, you would have to become a
Paid Subscriber since you don't meet the requirements of being registered for 1 year, have 300 posts, and have 150 reputations points.