Admittedly, I’ve never actually sat down and read a yellow “for Dummies” book, but I am still a fan, and I decided to borrow the idea for this article title. These recommendations come from my years of experience selling instruments at four music stores (two of them dedicated violin shops), many conversations with semi-professional and professional players, and practically half a lifetime of taking, and teaching, private lessons and public school instrumental classes. With the probable exception of #5 below (more wood means higher cost), these same tips can apply to purchasing violas, cellos, or really any orchestral string instrument.

Okay, here we go:

1) Don’t buy a violin on Amazon.
2) If you ignored #1, get out your wallet. Even if you're just "trying it out," chances are very high you’ll be paying twice just to get it playable! They do a great job making their packages look attractive, but be the one who read this post and doesn't get sucked in. Amazon instruments are a teacher's nightmare. Yeah, don't get me started. Seriously!
3) Purfling does serve a purpose; if the lines are painted on, that's kind of scary. Also, make sure the bridge has been properly carved: it should be completely flush with the violin top, not a bridge blank that comes separately.
4) You don’t always have to buy a new violin to sound better; try upgrading your existing instrument by buying a new bow or a nicer, newer set of strings.
5) Plan to spend roughly $300-500 for a decent entry level violin, no matter the size. This isn't as much of a surprise when you consider that a set of strings alone can be $50 or more, a new bridge is around $80, and a new bow or bow rehair start at $50. A block of wood alone for the top or back of the violin before it's carved can easily be $200 or $300. See post here.
6) Don’t be fooled by the label Stradivarius. Most real Stradivarius violins are either in museums or private collections and played by the best players in the world. They sell for millions of dollars.
7) Age is an advantage. Used violins are often a smart way to go.
8) Choose a violin first, then try out bows. Yes, that is real horsehair.
9) Explore the entire range of the instrument and ppp to fff.
10) Similarly, try many different bowing styles when trying bows, like spiccato, up-bow staccato, sautille, marcato, and legato.
11) If those words in #10 looked like Greek to you, if #9 had you wondering how, or you otherwise aren't sure what to look for, don't buy from a private seller unless you invite an experienced player to tag along. Choose a reputable music store, where the personnel are highly trained to guide you through the process!
12) Know your price point ahead of time: compare apples to apples. You wouldn’t try out a $3,000 instrument at one store and then try to compare it with an $8,000 instrument at another store.
13) What you’re paying for: Just like buying fine art, you’re always paying more for a famous maker, better materials, more carving and care, etc.
14) Particularly when moving up to an especially nice violin, including what you may consider your “final violin,” try out the few final contenders in different acoustical settings.
15) The hierarchy of professional violin levels generally starts with Italian at the top, followed by French, followed by German.
16) In my opinion, wood bows “breathe” better and will always trump synthetic bows, though it's true that carbon fiber bows are becoming more well-made as time goes on.
17) It's nice to have someone listening to you play on a potential violin from a few feet away, because violins can sound quite different under your ear. When my parents bought me my first full size violin, we got permission to take some [similarly-priced options] to a church, where my mom and teacher laid down on the benches so they couldn't see which violin was which while I played on them. They were then able to offer their comments without prior prejudice.
18) If you are a college music major who aspires to play professionally, plan to spend the same amount on your instrument as you would on a new car. Some professional symphony members pay more for their violin than their home mortgage. That being said, it is an indisputable fact that how you play the violin far outranks what violin you play.