In the past, expired domains were used as part of black hat SEO in order to generate backlink sources quickly and easily, and this practice is still used somewhat today. Getting links in this way is not a recommendable practice. All relevant search engines work nowadays with highly complex ranking algorithms, which can no longer be outsmarted by these link networks.
Yandex replaced TCI with SQI (Site Quality Index). SQI looks similar to TCI, so I re-used the old TCI field, however all values are now the new SQI. Not all domains are checked yet, but all that had TCI > 0, are now switched to SQI and all newly added domains are also checked for SQI (starting today). Updating all domains will take a couple months.
Just because someone spent five or six figure amount on a domain name doesn’t automatically imply that he/she’ll spend the same or at least a few thousand dollars on a similar name too. Basing your domain flipping strategy entirely on what others have just bought may not be as useful as you may think. Every domain name has a story behind it and its purchase/sale is usually as unique as that story.
There’s an important distinction to make here between domain flipping and website flipping. The latter mainly refers to buying and selling full websites. By full websites, I mean websites that actually have content and more often than not have traffic and revenue. When buying and selling websites, the domain name matters less as the main value there is the content, traffic, revenue, history/reputation, sustainability and growth opportunities. The same can’t be said for domain names where you’re just selling “the name” and hence it’s all that really matters.