When pets are in a rancher shop, they freeze. THey will not age or get hungry.
As for pricing, try the "Search Pets" section under "Play" in the navigation to your left. Narrow your search to whatever kind of pet or color you are looking for in the rancher OR user shops, depending on what you are looking for. Rarer pets, colors, and genders are usually worth more.
Remember: Rancher Shops are for creatu; User Shops are for items, including eggs.
Hope that helps. If you need further *censored*istance, please contact me or a staffmember.