OP, your prices of domain registering are not particularly attractive. There are already more trusted services with lowerest price, also accepting bitcoin.
I want to buy a domain name, which is already registered by someone, but one thing I am sure that he's not going to renew that after it gets expired. so do you have any feature like when the domain is available again to register, I got notified or something like that?
Depends on which registrar is used for this domain registration. I know, Godaddy put all expired domains to auction, usually, if this domain has some backlinks and traffic, someone will buy it and redirect to the domain where they want to improve SEO.
Redemption period is usually 30 days.
That's double profit for GODADDY, investing in someone's effort to create traffic and backlinks, webmaster should just sell their own domain that has a good traffic and backlinks, it's not easy for a site/domain to create backlinks and traffic, only for GoDaddy to get the opportunity to sell it, anyway every webmaster has their own reason why they let the domain expires.