There are a lot of things to consider before buying but I think buying a used boat will save you a lot and I mean a lot of money. Most boats have a tendency to depreciate in value. I well-kept used boat that is several years old may be in better shape than a used boat that is only a couple of years old. Something to consider when buying a newer used boat is it may have some warranty left if some major work has to be done.
Getting it checked out by a reliable mechanic in the shop is the first route I would go. If there's a problem, he'll find it first and give you an idea of how much it would cost to fix. You can also use this figure when you try to negotiate the price. You won't be telling the seller what you think it is worth, you'll be able to show on paper how much more you will have to put into it to get it running right.
Knowing the seller can help but is no guarantee. This can strain a relationship between friends if you feel you've been taken advantage of. Make sure to ask questions, has he had any problems with boat, does it take on water, electronics, pumps, livewell system, when was the last time a new water pump was put on, does trailer lights work, etc.
Beware of a boat that has been sitting for a long time. One of the first things you would probably need to do is drain the old fuel and replace with fresh. Someone made a good point that a boat that has been used is much better than one that's been sitting.
One of the cheapest ways to get a good idea is to go out with the seller and put it thru it's paces. See how quick it comes out of the hole, how does it handle on the water, listen to the motor. Make sure to run it long enough to get it hot to see if that makes a difference in a motor.
I shied a way from a boat from a guy I knew that had a hiccup every time he started to plane off. It indicated to me there was a problem that was going to get worst down the road. I ended up buying one that was 1 year old and paid almost twice what the other boat would have cost. I paid more but felt like I got much more boat. The bad news was it may have hurt his feelings because he hasn't had too much to say for the last few years.
If possible, it is usually best to pay more (if you can) and get what you want than to buy now and 2 years later wish you had something else.