SEO experts say godaddy is best for buying domain, godaddy is also best domain seller in world so you can buy a domain from godaddy.
When you are choosing domain extension remember few thing Your purpose and your country Domain.
and don't forget keyword in domain.
And last thing is make your domain name short catchy and memorable.
There are so many registrars that offer for cheap, but you have to understand which is easy to register or renew your domains. If you have a Credit Card then you can buy anywhere for getting more discounts else have to choose from local providers.
I would suggest you to go for GoDaddy as I too have purchased domains from GoDaddy and find it useful to manage domains with easy going steps.In case if you buy domain only plan, you may also upgrade the services anytime which do not require any further support. The themes that is being provided by GoDaddy, are so attractive that even being free they are referred more. From Buying domain to using the services for web hosting GoDaddy, is really a good platform.
Well, i suggest you to go for auctions.godaddy.com pay 5 bucks for annual subscription and enjoy 12 months fishing the domains. This is the best place where you can really find your domains because it has a large numbers of domains available especially those are going to expire, so got you another few bucks.
Flippa, Sedo are different marketplace and you need to have pocket full before you come here.
I know Flippa has a lot of scam sellers that use fake income and traffic proof screenshots to attractive people to bid on the listing just so that they can make money. You should let it run for a few days on your hosting and confirm that it is making money before approving the payment on escrow.