Getting your best deal on whey protein is easy as 1-2-3 

One. Read the ingredients.  Whey protein concentrate or isolate (or a blend of both) should be the first ingredient(s).  Whey isolate contains less fat and carbs than concentrate; but the difference is not enough to justify paying extra.  Besides, there is growing opinion that the extra carbs and fat in concentrate slow it down in the digestive tract, which helps you get the most muscle-building bang from the amino acids.  The amino acids are the reason whey is so popular with strength and fitness athletes.

Two. Look at the "Supplement Facts."  More than 4 grams total fats, 8 grams carbohydrate, or 80 milligrams sodium per-serving is pushing the limit.  The higher those numbers the lower the quality (the less protein you are getting).

Three. Still looking at Supplement Facts, divide the grams of protein per-serving by the total serving size.  Example: Serving size is 30 grams.  Protein per-serving is 25 grams.  That yields 83 percent protein.  The higher the percentage the better.  Below 60 percent is a loser.

If the eBay seller doesn't list the ingredients and Supplement Facts, you can find them on the the brand's website.  There are other comparison techniques you could use, but the percentage method is the only one that shows you how much protein you're getting.