1915 D/D Buffalo Nickel

I would not trust the PCGS price guide for anything below MS grade. For most modern coins, sub-MS price guides seem to be based on a percentage of the MS60 price estimate, not any actual data. In your D/D example, there are two recorded HA auctions for F10 at $182 and $259, but the PCGS price guide estimate is still $85.

Great Collections has five 1915-D/D 5c examples in the past 10 years: VF Details ($54), VG10 ($64), F15 ($87), F15 ($118), MS62 ($2145). Be aware that searching for D/D only yielded 4 results. Searching for FS-501 located all five, so make sure to search both text variants on the other auction sites.

ebay currently has an auction running for an ANACS graded coin:

https://www.ebay.com/itm/134947664947

Also a recently sold AU example on ebay for $1000:

https://www.ebay.com/itm/364715320420

Another $1000 current ebay auction, no pictures of the ANACS slab, from a seller with four prior sales (scary!):

https://www.ebay.com/itm/355498037554

The same ebay seller, a $900 sold version of what appears to be that same coin. Relisted? Bizarre.

https://www.ebay.com/itm/355495515084

I have a PCGS graded AU55. For the life of me can't remember what I paid for it , but I know it was north of $500 and less than $1000. A tough RPM for sure!

Heritage has sold 15 of them over the past 24 years, just another data point. PCGS has graded 31 and NGC 9. Highest grade for both is MS63.