I've never sold any so I have no idea.

A local coin shop (LCS) or online (usually a little better price) is what most folks do. AGEs, AGBs and ASEs are all I suggest buying as they are a known quantity and tough to counterfeit and fairly easy to authenticate. Most place will give you spot for the Au coins mentioned and 1% over spot is not uncommon. Again, somewhat influenced by the metals markets.
I have not priced AGEs or AGBs lately but a few years ago the buyer's premium was roughly 5% if you shopped at the right place so if Au spot was $2,000, then you'd pay $2,100. Premiums can be as low as ~4% or as high as 10-ish% for Au so shopping for the best price is a smart. The Au market (availability, price, refining capacity, etc.) also influences premiums.
Ag has always had a much higher premium for ASEs, usually in the 10-30%. This is why I stayed away from Ag a few years ago. There's also the storage factor for Ag as it was roughly at 80:1 then. Now it's better since Ag has been up a lot. Now, it's 60:1 so for every ounce of Au you have to store, a like value of Ag is 60 ounces.
Buying a precious metal for the value of the metal (or coin in this discussion) is known as "stacking." A few scratches or handling marks on a AGE or AGB does not effect the value.
Buying a coin for its collectible value is known as its numismatic value.
I would suggest staying away from the latter and stick to the former. Much more certainly in metal values with stacking given the markets.
Let me know if I missed anything in my reply. I'm not trying to convert you to physical Au but do want to answer any questions I can.
Eddie