You can find out what pets are going for in the market using the Advanced Creatu Search (ACS) located in the navigation bar. Select rancher shop for location and the pet you're looking to find the price for. Make sure you put the price as 1 to 10000000000000000 or something like that and sort the results by price. Click search and it should show you what they normally go for. You usually sell it around the cheapest avalible one, or higher if the name is very good or a RWN. I think the rarest colors that are hatchable are in this order, from most common to rare.
-Natural
-Sepia
-Cream
-Black
-Ginger
-Silver
-Blonde
-Albino
The other colors which are either at the price of Albinos or higher/slightly lower are dye kit colors. You buy them from the Black Market located in the shop lists at the navigation bar or from user shops which you can access by typing it into the search bar at the top. If you're going to search in user shops, I reccomend first typing dye kit to see the types of dye kits that are being sold. Not all pets can be painted all the dye kit colors. You can see the colors that each pet is avalible in here:
http://www.rescreatu.com/hatchery/creatulist.php
The lower the rarity of the egg, the higher you can sell it for. You can check the prices of the eggs the same way you would try to find the price of a dye kit- the merchant shop search. If you're lucky, you can hatch a rare hatchable color, but if you're not, you can save up Tu which can be made with games and buy them from users' rancher shops using the Advanced Creatu search. Here's a list of the rarest dye kit colors from common to rare. The prices are always changing though, so check up in the merchant shop search to see how much people are selling them for.
-Lemon
-Lime
-Magenta
-Orchid
-Azure
-Rose
-Gold and Silver (commonly abbreviated as GaS)
There is, sadly, no list of pet values, and the best way to find out the price is to use the ACS. If it's really rare and there aren't any pets at a buyable price (some people put pets in their shop at an unbuyable price which is a bunch of random numbers in the billions), then I suggest going to the ad forums (you can access the forums at the navigation bar) and setting up a thread saying you're selling that pet and looking for offers. You can do that even if the pet you're selling has a specific price on the ACS. The ad forums are for any advertisement really.
I hope I helped you out. Feel free to Rmail me if you need any more help. c: