Purchasing soccer goals can be a fairly simple process. It's just necessary to understand what features you are looking for, and what uses you intend to use them for.
First you must decide how you are going to use your goal. Will it be used for practices, or actual soccer games? The difference between the two goals can be quite different, depending on where you set your price. Practice goals can be purchased for cheap. They are plastic, and come in many different sizes. The advantage to this is they are easily maneuverable throughout the soccer field, or you may simply take your goal to whichever field you wish to practice on. This is very advantageous in that aspect. The disadvantage to plastic goals is that they are much more poorly built than more expensive metal goals. Plastic goals are hard to keep still, and most need to be anchored to the ground. They are also usually smaller than actual sized goals. But nevertheless, plastic goals are much cheaper and can be used for practices on any field. They also have features available such as a tight net to bounce the ball back, for practice without a goalie.
The next major type of goal is the metal goal. These are fully sized, very heavy, and difficult to take from one place to another. These are most commonly used in actual soccer games, rather than practices. The advantage to these is that they are weather resistant and very sturdy, but must still be anchored to the ground. The primary use of these is for soccer games or more permanent locations. There are also multiple metals to choose from, including aluminum and many others. This makes no significant difference, it may just affect the weight and durability of the goal.
The last major thing to worry about when purchasing a goal is the various features or accessories it may come with. This depends solely on what you prefer or how much you would like to spend. For example, many of the soccer goals come with anchors, so it may be easily anchored to the ground. There are also various net materials, depending on what you would like. Depth is another major factor for the goal. Depending on your purposes, you may want to purchase a goal that has a more or less deep frame.
Purchasing a goal should be simple. All you need to do is know what purpose it will serve, and what features you are looking for.