If you purchase a used pickup truck or even a used utility truck for commercial usage, it is going to be deductible under Section 179 (Source: Used Utility Truck by Hengehold Trucks). You will be able to take the full purchase price of the truck and deduct it from your taxes. It is important to note, however, there is another consideration which is known as bonus depreciation. This is only going to be allowable on new equipment, including new vehicles. If you are a large business and spend $560,000 or more annually on capital equipment, you may be able to take advantage of the bonus depreciation on new vehicles that are purchased. You will not be able to use it on used vehicles.
There are always considerations when purchasing anything for your business. Talk to your accountant before making your purchase to determine if it is going to be to your financial benefit to purchase something that is used. You will likely find that it will be beneficial.