Selling car seats is a double standard that you will often find here. We are all told not to buy used seats but we are all encouraged to sell the seats that are no longer working for our situation.:shrug-shoulders:
Basic guideline for pricing:
If you recall what you originally paid for the seat you take that amount and divide it by the number of years it can be used (6 years for most car seats, 9 years for the Britax Frontier, 8 years for the Radians, ect). If you are not sure find the *Do not use after December of 20XX* message on your car seat shell. Some (like Britax list it on the label with the model number)
Once you have the cost per year multiply that by how many years are left in its life span.
Here is an example:
I have a Britax frontier85 that retails for $200. It has a 2011 DOM and a 9 year lifespan. So $200/9 years=$22 per year. It has 7 years of life left (give or take) so multilpy $22 by 7 years= $154.00
That gives you a starting point. Craigslist is a good place to start but folks there are usually looking for less expensive seats. Depending on how quickly you want to sell the seats you could drop the price even more. List the DOM in the description so that prospective buyers know that it is not an expired seat.
Now for responsibility, make sure all the parts are there, especially the users manual. You might mention if/how you have washed the cover and if you have washed the straps (which you should not do).
Hope that helps