What is a domain purchasing?
The search of domain name options runs as it follows. Every time after you request for some domain, the domain registrar queries more than 500 different sources. Shortage of speed can be caused by exclusion of domains that have been reserved by the original provider.
There is no offers of premium domains because it doesn't have a big sense. You can use a lot of cheap options for domain names obtaining.
It is possible if you come across premium domains, but in order to register them, you need to contact directly the TLD registrar. It happens if you search for widely-used words or names. They are often reserved by registrars to sell as a premium.