So yes, it is legal to re-sell and buy old licenses, and it is technically straightforward to transfer them to a new computer (if you have the .key file etc.).
But it will not be straightforward to find a license available for sale, I'm afraid. I have seen a few enquiries here and on other forums, none of which got a positive response. Seems that those users who have not simply forgotten about or lost their old Eagle license want to hold on to it -- as "insurance" for accessing old designs when needed, or even for new designs. I am certainly holding on to my 7.x license (hobbyist license only, i.e. not what you need), even though I have switched to KiCad for new projects.
Yes, that's quite right. I need to hold onto my 7.x Eagle Pro license to work with old but well-proven designs. And like you, new designs will be done in KiCad.
The Eagle import into KiCad is not so bad lately, but when a client asks for another 100 boards with a couple of tweaks, I'm not going to risk an import, edit, and then off to the fab. Any board errors because of MY decision to switch will not be received well.
There could be 7.x or older licenses floating around as a result of users who fully bit into Autocad/Eagle 8.x and later. If Autocad/Eagle was considered an upgrade to a 7.x or older version, then probably not, but if they had to buy into Autocad/Eagle fresh under the new rent-ware model with no regard and no value offered for a previous license, it would seem to me the user fully owns the old license and could resell it if desired.
A read of the relevant license agreements would be needed.