question How good is it to buy an expired

It is worth the merit of the domain itself I let domains I don't want expire if don't sell on a 7 day auction. If i drop a domain it is probably not in my regular niche most if not all are picked up.
The advantage of expired the domain may have been developed with back links, the quality of the domain maybe better.
Expireddomains.net has some pretty good data.

Hello Brian,
imho, you can forget all the tools to know if an expired domain is clean or not. Moz and majestic give you a spam level indicator, which is a poor as their DA and TF. Some use a ratio such as TF/CF, but it makes no sense. 95 % of my colleagues are using complex algorithms and think that they'll be able to answer their needs. To me (I'm surely wrong, as I'm in the minority side), it can never work. In this case, the only CPU that works is the humain CPU.
So what I do, for every domain I check, is:
1. screenshots.com. In two seconds, my eye will remove at least half of the pre-selected domains. Then the anchor texts (ie. in majesticseo) and 10 % of the remaining domains will be abandonned also. And eventually, because it is what takes more time, archive.org, which makes me remove another 10 %.

This thread just caused me to buy.

Penalty you check in google, site: operator. Spam you check in Ahrefs and archive.org.