Is Building A Boat Better Than Buying A Used Boat? 
First, let us take a look at what constitutes building a boat. 
You could build one starting from scratch, that is, obtaining plans and designs, purchasing materials and tools, investing a lot of time and effort. Depending on the size of the vessel, it could be a five-year project. You can buy a boat kit and assemble the pieces together and fit it to your choice. Not as time consuming as the first option but is going to cost you more. You can also have the hull/complete boat made by a professional builder. This will still involve deciding on the type, design and size so that the builder can give you an estimate on how much it is going to lighten your pocket by. 
Naturally there are pros and cons to each method that can't be discussed in detail here. 
The other option you are considering is buying a used boat. As yo are trying to compare building versus buying a used boat, I'm thinking cost is a big factor. And here, I have a surprise for you! Sometimes, a used boat may actually cost you the same or just marginally less than a new boat. Depending on the type and model, a used boat may even cost you more than a new one! A used boat will come with all the additional fittings that the previous owner has installed and if it is in good condition, a buyer may be ready to give him more than he will pay for a new vessel sans the extras. Just like buying a used car, you may need the help of an expert to assess if the boat passes muster. 
I'm playing the devil's advocate here, but it's really going to be your call and yours alone. If you choose to build, be sure of your commitment to the project. You can't abandon it midway because there is no resale value for your effort!