The single most important thing about buying electronics from Ebay is communication!  Talk (via email) to the seller! Ask questions!  Get answers!  Know what you are buying!  Are the connections right on the item for your particular needs?  Does the product come with charger, power cable (for AC or DC auto)?  Is it in working order, or for parts?  What is the guarentee if there is one? 

Remember that electronics of any kind that are to work in conjuction with other components must match for use.  Know what you want.  Does it use a BNC connection or ? 

I have had great success purchasing electronics through Ebay.  The sellers I have dealt with were all ethical and very obliging.  Again, communication is the key to success.