You can sell pets in one of two ways, either directly to an NPC which gets you immediate money at whatever value the NPC deems your pet is worth or you can sell your pet via the marketeer system, the latter will no doubt yield more money but it will take longer to sell.
To sell to NPCs go inside Springlade 31, 59 and enter the Town Building, follow the path to the north and you will come to the Stable and the Stablemaster NPC, clicking on him will present you with the Pet Buy/Sell screen. Other towns may have stablemasters also.
To sell on the Marketeer system go to Minions > Price Pets, set a price for your pet and click the For Sale button, you will be charged a small deposit and then the pet is put up for sale, a pet for sale does not count towards your held pet limit.