The difference in a Fiddle and a Violin is the way it is played, and set up. A Fiddle Player wants to bow more than one string at a time while playing. A Violinist wants to bow one string at a time while playing. So in setting up this instrument, the Luthier puts a French curve on the bridge of a violin, and a flattened curve on the bridge of the fiddle. Hence, you have the ''Violin", and "fiddle". 

Years ago I was told you should always loosen the strings after playing this instrument. The only time you should loosen the strings is, #1. If one string is over tuned. #2. You are going to store this instrument for some time, or #3. If you are going to change strings. Now with that said, the strings when loosened to store, should only relieve a small amount of tension, you still want the bridge to stand up in place. The hair on the bow should be slightly loosened after playing, especially on a wooden bow, to keep the bow from warping or  bowing as the name implies.

A little history I aquired in my ten years as a Luthier. When setting out to make a fiddle, the wood is the first consideration. The wood used for the backs is maple, and Swiss white pine for the bellies. Both of these should be not only of the trees growing on the south side of the forrest, exposed to the sun, but also only the wood on the south side of the tree should be used. 

A little bit on varnish which I experienced. If you have a violin that is left in the sun to long and the varnish has seemed to melt and whatever it is touching has left imprints in the finish, you have a Spirit Varnish. You can French Polish the places out. Use a cloth made of old cotton, 100 percale, denatured alcohol and mineral oil. Put these on the cloth and work the varnish in a circular motion, using just enough oil to keep the cloth from sticking. If you keep polishing the bad places will disappear. This will take some practice. Other finishes are of a hard surface laquer and will not melt so to speak. So keep your eye open to the product you are buying. And also just because it has soft varnish doesn't mean it's an old instrument.                  KEEP ON E-BAYING!!!!!!