As far as I know, here are no legal limitations to sell a domain immediately after registering it... as long as you find someone to buy it. (there are limitations to changing registrar for 3 months, but it does not prevent the domain to be sold.)
Most domains are sold without traffic. If a domain is developped into a site with pages and traffic, it's usually called 'selling a website'
Your chances to hand register a valuable new domain are very small (but not zero). Most domainers consider that all valuable domains have already been purchased and they buy them from auctions hoping to resell them with a profit a few months or a few years later.