The rarity/collectibility of the coins I'm dealing with just adds another facet of 'intrinsic value' to them unlike bullion.
I haven't noticed anything but a steady increase in value/retail prices in them even in the face of falling silver prices.
The only thing I've learned that I have to do is not to offer too many of the same type/year coin simultaneously otherwise buyers know that if they miss one they can simply try again for the next one.
So for the ones I have many copies of, I'm spreading them out over time so as not to overlap the auctions. Instead I offer a variety of types and years, constantly recycling the listings as they end.
And the key of keys to getting a good price is the photography. I can actually buy a coin today with a so-so picture in the listing, photograph it myself with my 'patented method', and resell the very same coin for 50-100% more.