I have been working in the medical field for many years.  I have used MANY stethoscopes over the years--both my own, and ones I have borrowed from others.  You can tell a good scope from the way it works.  I have found that you get what you pay for--and that goes for on e-bay too.  I have purchased 2 scopes on e-bay, and have found that no matter what the starting price is, MOST good quality scopes, new and used, go for near their going price on the market--particularly Littmann's.  The nice thing about a Littmann is that you can get it re-furbished, and it is as good as new...you pay for the scope head more or less, can get all new parts around it.  The one way you can get a bargain is to find one with poor tubing condition, get it cheap, and change the tubing and earpieces at your local Littmann dealer. 

I would reccommend if you buy a used one, get new earpieces.  They are cheap, and easy to get.

To do good work, you need good tools.  Remember that.  If you can't hear, you can't give good patient care.  Get a good scope, it is worth the money.