First of all is what size boat the towers are meant to be mounted on. If you aren't sure what the dimensions of your boat are, get absolutely sure before you make a purchase. An improperly-sized tower can make your boat very dangerous to use. You want to make your measurements as exact as possible.
Before you invest in a tower, look into what is required to install it. Some are simple enough to be installed on your own without specialized tools or experience. Many are not. Understand what you're getting into before you spend anything. You don't want to find yourself saddled with a project you can't finish, and depending on your location and the season, there might not be anyone available to install your tower for you without a costly trip.
These are two of the biggest things you'll need to worry about if you decide to buy a tower online. It can be a very sound investment, but you need to get all the details first. Do your homework, and everything should be fine and relatively painless.