Here's what they've been selling for on Ebay-http://completed.shop.ebay.com/i.ht...hard+1+oz+silver&LH_Complete=1&_fsct=&guest=1 If you don't have an ebay account you can sign in as a guest. Looks like around 17 and up depending on shipping costs. Of course you should expect a discount when buying multiples. Engelhard is a good name and usually commands a small premium over spot price.
I would let him start the negotiation, by asking him what he wants for them. I have a few hundred of these myself and for some reason they do often demand a high premium. I would not pay a high premium though now if I were you. I got mine back when silver was cheaper, and I plan to hold these until I finally part with my silver. As far as one ounce bars go, they are probably the most in demand, as well as JM one ounce bars.
I would not pay more than 50 cents over spot.
Most dealers will not pay him more than that anyway if he goes there to sell. Some will offer him less.