Interesting Question. These rescue buoys seen in various forms used on American submarines from the 1930's and used till discontinued with the introduction of the Ohio and Los Angeles classes would have been tethered from the submarine once released. Their cables reeled out, the end of the cable would have by clever design ended up each over the escape hatches for alignment to guide down the McCann rescue chamber. Assuming a buoy was released or even was able to. These buoys were often welded securely after sea trials and may not have been possible to be released by human activation. During World War 2, submarines had these buoys deliberately welded down securely so they were not logged free during depth charging and reveal the submarine to the enemy. During the cold war this would have been continued.
These stricken submarines when they went down beyond crush depth and imploded likely would have released the buoys along with hundreds (thousands?) of other items in the debris field. If the buoys did let go in the death plunge, and still tethered from their cables they would have likely gone down too, their flotation lost when they too where crushed by the water pressure. Remember also, the escape hatch which by way of the cable reel would have been attached to the other end of the rescue buoy cable, the escape hatch in the very violent implosion would have been blown free likely too. If they blew free, then they might have made it to the surface. Likely if both the hatch and the buoy were blown free, it still would have gone down, the cable and reel tangled in the tragic mess.
Each buoy has attached to it a name plate telling the finder the name of the submarine and the significance of the find, and how to contact authorities. Could a name plate have been recovered from those depths? Possibly, but likely not. The E-bay find might have been an extra cast up by a shipyard worker 'after the fact'. If sold on EBay as authentic, then it was also likely a very sick fraud. The mold this name plate was made from could have been authentic, but was the plate an original from the stricken vessel itself? Likely not .... like many things on Ebay. I am not an expert here, only armed with a general knowledge of how the buoy mechanism works. I am skeptical. But a good question!
Other locating devises on newer submarines now exist, and the McCann rescue chamber is still in a more modern form still a viable if limited method of rescue if the circumstances are right. The DSRV rescue submarine and others like it in other Navies are the general direction of method now to address the much greater depths involved.