Know the Dimensions
Not all boats come in the same size so you must know the dimensions of your boat and look for a trailer accordingly. There are always some standard sizes so it should not be a big problem.
Be sure that the trailer that you purchase has a good weight capacity. Ideally it has more capacity than the actual weight of the boat. This is generally believed to be beneficial in the long run as the trailer lasts longer this way.
What kind of environment are you going to be boating in? Whether it will be fresh water or will you mostly be going to the sea. This is an important question to answer as trailers are generally made for specific environments. Also, make sure that the trailer is rust coated and will not be damaged by exposure to water.
Have a budget in mind. The price will be dictated by a number of factors including the condition of the trailer, whether new or old, the material that is used in its manufacturing and obviously the manufacturer. The new trailers will certainly cost more whereas trailers that are used or are made by relatively less known manufacturers will not be as expensive.
The trailer will be attached to your truck. Make sure that the hooking mechanism in the trailer and the truck match. Although these are generally standard in both trucks and trailers, exercising caution is never a bad idea.
Shop or Online
Everything that is available in a shop can generally be bought online. Compare prices and options and make sure that you purchase the trailer from one that offers you the best value. If ordering online saves you money in the end of the day for the same product, it’s the best option.