Although civilization falls, people still has the same needs. Several resources are crucial and will have good trade value for your base. Beware that a resource-rich base will also be a target for bandits and throngs of random people seeking shelter that will, as a result, attract zombies.
WaterA clean source of drinking water has been essential to all societies since the first humans evolved in Africa, so it will be the start of any post apocalyptic-society. Whether you own a small well, desalination plant or oasis, people will always need you in order to survive. You can even buy water and transport it across the land to people who have no access, creating a virtual monopoly in that area. But proceed with caution; raiders may target you for your valuable asset, so traveling in well-armed groups would be a wise choice if you wish to conduct repeat business.
With most men currently burning between 2000 and 4200 calories a day, they will need food. A LOT of food. A rich soil has always been one of the largest factors in our cradles of civilization and will be priceless asset in a base. As the same with water, surplus food can be stored in times of siege or barter it for other resources that you are lacking in.
Oil, petrol, diesel, gasoline, ethanol, biofuel, or whatever you call it, is power. It's the power that drives the engines of cars, and those of generators that provide people with the power to run appliances, lights, and machines. Weather it's in the ground below your platform, in the tanks of your refinery, or in the grease trap in your grill, it's going to become very valuable. In fact, more and more Americans are switching to bio diesel, a cleaner, renewable fuel that is able to be produced by the average person, and usable in most diesel powered machines. This fuel may be the only thing availiable after all the other stuff is gone.
Metals like gold and silver will always hold value, but they will only make you a target for robbers in the beginning of the panic. Gold has been desired by humanity since the dawn of mining and will likely be useful in an aftermath scenario, but has no use in a zombie apocalypse world apart from sentimental and as an electrical conductor. Therefore, unless you're an idiot or an electronic genius, don't even think about gold. Other metals like iron, aluminum, and steel will be required for reinforcing defenses, building roofs, and making weapons. They will be at a higher demand than the "precious" metals.
As time goes on, ammunition will become scarce, and that which remains will literally be worth more than their weight in gold. Anyone with access to stockpiles, anyone with the knowledge to operate a reloading bench or make gun powder will become invaluable to survivors. Ammunition means protection. Ammunition means fresh meat on your dinner plates. Ammunition means survival.
Guns are worth more than gold. Gold can't protect you, or your family. Gold can't put food on the table, gold can't kill a zombie. A gun can do all of those things. A gun can become your biggest survival multiplier. If you don't want one? Still, keep it. It might just save your life. Still, if you really have issues with firearms, a knife, machete, or sword might keep you alive just as well (provided you're in good shape, don't tire easily, and actually know how to use a melee weapon). If you don't want either, than take a zombie weapon and get a bow or crossbow with some arrows.
Tools maintain your house, maintain your guns, maintain your machines, and maintain your way of life. Tools are one of the key elements to rebuilding. Without them, nothing sustainable can be built, and nothing built can be sustained. Tools are what keep everything manmade operating, and can be used for a very long time if kept properly. Whether you're a tinker, carpenter, or mechanic, the future will need you, and your tools.
You can't survive an apocalypse just wearing the same dirty shirt you wear to the gym. You need to have clothes that last the test of time. Having high quality military grade clothing is essential for survival. Clothing provides the first line of defense against the elements.
When disaster strikes the first 72 hours are the most essential. Leaving urban areas with higher population is vital to staying alive and safe. You can't complete the journey without having supplies for the first 72 hours. Having a great zombie survival kit ensures that you have everything you need to make it out of the city while not worrying about what to eat or drink.
Knowledge has always been humanity's greatest asset and what sets us apart from the zombies. It allowed us to create everything to keep humanity one step ahead of the undead.