It can be so tempting! An ebay deal on a violin for $15-$50. Why not?
The set-up and sound quality of an instrument is what matters. Not the year it was made. Not the price tag. Not even it’s pedigree. Like anything else in life, you will usually get what you pay for, with a few exceptions.
Having an instrument with correct proportions of the fingerboard, bridge, soundpost, and string height can make the difference between a student hating to play or loving to play. The sound that an instrument makes is also a quite significant factor.
Instruments, like artwork, are good investments. You can re-sell them for what you paid for them or more (as long as they are not severely damaged).
I try not to get overly political about where you get your instrument. I teach at Kirk’s House of Music, and I do really like their instruments, service, pricing, and generosity. I also think that it is very ethical to support local music stores and keep them in business. However, if you chose something else, that is fine. I do hope that you will check with me first though – do not just purchase a cheap deal off of the internet or from a pawn shop without knowing that it is returnable. Otherwise you may end up regretting your purchase.