I've seen this happen a lot. This is how it works - scammers post high value links to a worthless domain thereby getting the PR of the domain to increase. They then put the domain on auction once the PR is high. Eventually someone buys this $10 domains for a ridiculously high price. They remove all the links, and start over with another worthless $10 domain. In the next PR update the PR crashes as the links have been removed. Since PR updates do not happen regularly, it takes a long time for the buyer to know he has been scammed.
What you can do to not be fooled -
1. Check the back links of the top 10 domains (check in Majestic Seo and Ahrefs). Look for all contextual links i.e. links that are surrounded by relevant text. Ignore site wide links (blog rolls, footer links etc). If you have enough high value contextual links, then the domain will retain its PR.
2. Check the last site in Wayback Machine. If you see the site to have suddenly changed (UI changes, content changes), then its possible that someone already bought this and tried to make it a part of a blog network.
3. Check the anchor text distribution in Majestic Seo. If you see any scammy words i.e. keywords not fitting in with the niche, then this domain may be a scam.
4. Check whether the domain is being indexed by google using the "site: domain" and "info: domain" search queries. If google has not indexed any pages, the domain may be facing a manual penalty. It tends to be very hard to get such a penalty removed and till then the domain is worthless.