A few added points on buying silver nickels based upon the silver content.
1. Weight. The listed weight for silver nickels is generally 5.0 grams, as minted. This is the number used in silver content calculations. However, based upon my recent buying experience of general well circulated silver nickels being sold on e-Bay, I find the gross coin weight to be 4.7 to 4.8 grams. This is reasonable wear of 4-6%. However, if one is buying the nickels for silver content, you will want to factor the lower weight.
2. Non-Silver nickels in larger lots. When buying lots of 100+ unsearched silver nickels on e-Bay, I've found they generally contain a reasonable error rate of 1% non silver nickels. One should also consider this 1% error rate when calculating the total silver content of silver nickel lots.
If in doubt, ask the seller. I've found those selling silver nickels to be helpful and responsive to my multiple questions on their lots for sale. If you have questions, just ask them.