I don’t have an easy answer. Amazon was the 1 ton gorilla in the room so that was the one I was familiar with, but I will try to update this comment as I find information out.

Kobo: Seems like it’s yours to keep without resale or transfer rights. (https://download.kobobooks.com/learnmore/kobo1_pdf/Kobo_eReader_Terms_of_Use.pdf)

Google: Yours to keep provided the authorized agent and Google themselves maintain rights to provide it. (https://books.google.com/intl/en/googlebooks/tos.html)

B&N Nook: seems to be more in line with Amazon (https://www.nook.com/services/cms/doc/us/en_us/legal/nook-store-terms.html#ItemsPurchases)

Smashwords: seems more like ownership without transferability. (https://www.smashwords.com/about/tos)