Yup, I always, always, always simply check for domain availability via pinging the TLD’s registry directly using the whois command in my terminal. I’ve been burned like this before by checking on another registrar (I forget which so don’t want to make accusations) and not getting past the checkout process like OP. And with the whois command you don’t need to worry whether the registrar you’re using to search sells the TLD you’re looking for or not, which is a nice plus.

There’s absolutely no reason to ask a third party domain registrar whether a domain already exists. People need to spread the word more!