Sure, domains consisting of a mix of numbers and letters are sold and developed, but the domains have to have some meaning or be brandable. E.g. a domain such as tokyo2020.[com,.org,.co.jp] will have value because it has a meaning (The Olympic Games are taking place in Tokyo in 2020), and w3c.org is used by the World Wide Web Consortium. Generally, domains with (randomly) mixed letters and numbers will have little to no reseller interest and no end-users interest.
The only exception is two or three character .com domains. Any combination of letters and numbers will have a resale value of mid $xxx and up.