Your entry for Omnivore says the link to the hosted version is broken. That's because it shut down at the end of last year.
It was a good option for read-it-later, whereas most of the of the bookmarking self-hosted apps seems more focused on saving and sorting links.
If you buy from a reputable seller with a good free return policy, at worst you'll have to send the device back. If you have access to a credit card or PayPal, it's a good idea to use them, in case the seller becomes unresponsive.