kits

Buying and selling is not allowed on the forum. But there are other sites where you can, I am sure someone will give you details. I have no idea which ones to us myself.

Yea what he said,some other sites that allow selling are

www.armorama.com

www.modelerssocialclub.proboards.com

www.modelersalliance.com

check em out or try ebay or local model shows

In my experience, second hand kits are generally not valuable to others, and not worth the trouble to sell.  I would rather donate them to a veterans organization, or a local charity that gives Christmas gifts to kids, or whatever is  your preference.

That depends on which kits they are.  Some are desirable and some aren't.  Check online prices to gauge the selling price.  Also, check your local model club (is there an IPMS group in your area?) to sell locally.  There's always the local buy n sell (craigslist)