Collections are a group of items that can be traded in for a bonus, usually Coins or experience. Be aware that many collection items are needed for Quest completion or Weapon Crafting, so DO NOT trade them in immediately. My rule right now is to have at least five of each item on hand. The collections correspond to the zombie type that drops them. You can find how many items you have in a given Collection in your Inventory. The number you have is also shown in the Weapon Crafting windows and Quest boxes if it is needed to craft that weapon or complete that quest.
Supermart Zombie: 500 Coins[]
Motor Oil, Toy Gun, Rope, Toilet Paper, Candy
Salesman Zombie: 25 XP[]
Baseball Bat, Boxing Glove, Tennis Ball, Bowling Ball, Football
Janitor Zombie: 1000 Coins[]
Metal String, Alarm Clock, Mop, Lunchbox, Rubber Glove
Gas Station Zombie: 1500 Coins[]
Gasoline Can, Propane Tank, Lighter, Car Battery, Hose
Waitress Zombie: 2500 Coins[]
Diet Cola, Liquor Bottle, Dry Ice, Dinner Knife, Salt Shaker
Plumber Zombie: 40 XP[]
Suspenders, Spray Bottle, Metal Pipes, Plunger, Rubber Duck
Gardener Zombie: 80 XP[]
Red Peppers, Rake, Potatoes, Garden Trimmer, Fertilizer
Firefighter Zombie: 3000 Coins[]
Fire Extinguisher, Flashlight,Matches, Boot, Metal Bucket
Construction Zombie: 5000 Coins[]
Nails, Helmet, Tape Measure, Paintbrush, Traffic Cone
Doctor's CollectionHead Mirror, Otoscope, Knee Hammer, Medical Kit & BP Monitor (Trade in for 90 XP)
Nurse CollectionPills, Kidney Tray, Syringe, Thermometer & Baid Aid. (Trade in for 6,000 coins)
Dock Worker CollectionWrench, Broken Nail Gun, Small Motor, Welding Mask & Buzz Saw (Trade in for 3,000 Coins)
Sailor CollectionLife Float, Fishing Pole, Harpoon, Diving Tank & Rucksack (Trade in for 90 XP)
Miner CollectionDynamite, Mining Tools, Mining Helmet, Mining Lamp & Coal (Trade in for 3,000 Coins)
Cowboy collectionHorseshoe, Spurs, Bandoliers, Gunpowder Pouch & Bet Buckle (Trade in for 5,000 Coins)