Why buy a used car or boat?
Because the worst investment you can make is a "depreciating asset", meaning something that loses value over time.  Example: if you buy a new truck for $35000, in 2 years you would lose about $15000 to $20000 in value, when you could have bought 3 5-7 year old trucks for what you would have lost. If you buy a 6 year old truck for $7000, and use it for 2 years it will still be worth $3000-$4000 and even with an increase in repair costs you would save thousands. 
Why buy from a small, independent dealer?
Because they are probably a neighbor, and member of the community, and not an out of town investor, and the profit made will be recirculated in the local economy. Also, the cost of doing business is lower, resulting in lower overhead, therefore allowing them to sell a similar car for less than "the big guys".
Should I buy a warranty?
That's up to you for your level of comfort. Let me give you some facts, however. If you can buy a warranty for 3 mos. and 4500 miles for $250 and the warranty company is showing a profit, how many claims are filed? A warranty is like life insurance, you only win when you lose (think on that one for a sec.).
What about "high" miles? 
Most of the warranty companies I use will offer a warranty on vehicles that have up to 175K miles on them for as low as $150 for 3 months and $595 for 12 months and unlimited miles. If that was "too many miles" they could not afford to do it. I usually drive a vehicle with over 200K miles myself just to show that , if taken care of , a vehicle with miles on it is still a good, servicable item. A Chevy truck was just "retired" in the midwest with ONE MILLION miles on it with the original engine. The owner used it for deliveries for it's whole life, that's "high miles".