Well, there's 2 questions as I see it.
1. Is it copyright infringement.
Yes, it is.
2. If one were to list one of these tapes on ebay, would they get busted for copyright infringement
Possibly, but not likely.
I think that people selling blank tapes that have materials recorded on them, while technically copyright infringement, happens quite frequently on ebay. I actually have a few open reel tapes that were blanks with stereo material recorded on them, that came with something I bought off of ebay. However, this is a little different, since the rarity of the material does result in the value being significantly increased when it comes to the Robin reels. I would guess that a few Mike Robin reels appearing on ebay would sell for big money, and that would be the end of it. Of course, the large dollar amount could possibly get someone's attention. Or, if one were to start duplicating Mike Robin reels and sell large amounts of them on ebay, that would certainly increase the chance that someone would notice and take action. But, selling a few originals, probably would fly under the radar.