Almost ANY circulating item in the Marapets database can be a Quest Item and some retired items as long they're cheap on the Shop Search. A Quest in Marapets is a game that requires you to search for certain items and you need to bring them back within a time frame. Quests usually ask for 1 to 3 items that can sometimes be easily found in the Marapets main shops. When the main shops are out of the item you can use Shop Search to find the item in another user's shop.
This is an example of the items that may be asked for in quests. Some of these items may also be asked for in Missions as well. To see the items relative to missions, take a look at this page.
TOYS & PLUSHIES
Description: Toys are very useful in many ways, you can give them to your pets to play with for happiness, and to add to their toy collection. Toys are often asked for in many quests, but the one quest that will ask for the most toys is the Santa Claws Quest of Biala Mountain.
Description: Candy Fruit are possible Candy Tree prizes ranging from 10,000MP-280,000MP on the Shop Search.
Description: Chocolates are requested by almost every quest in the Marapets game, and it is perhaps one of the best items to restock. The Candy Tree in Candyland north of Chocolate River loves to ask for chocolates too.
Description: Marapop cans are good for giving energy to your pet when it gets tired. They are also a very popular item in many quests (like Hump Racing). When you feed a marapop can to your pet, you get an empty marapop can that can be recycled to get wonderful prizes at the Recycling Centre. The Tombola can give you some unlabeled marapop drinks, which are very popular too.
Description: Computer parts are used for many quests, but mainly for the Computer Repair Quest  in Ziranek. Computer quests are useful for earning magic for your pet which is useful for healing in the Battle Arena.