Many students are given personal pages on .edu and they can sell or link to anyones website. This will happen in future.
That is left up to the institution that owns the .edu. If they allow their students to sell links like that which occurs in other TLD spaces, that would be their choice and I do believe they may end getting themselves into something that may potentially be a problem. Allowing this to happen leaves the TLD open for poison and I don't think any credible institution is going to allow their students to do that, not in the US anyway and not on a scale where it is as prevalent as other TLDs.
And also many small colleges in my country are alloted .edu. Their management is not trustworthy as US universities. If you approach them for link renting, then they will do with pleasure. Please note that .edu is being alloted to any educational instituion in the world and not USA alone. Many schools and colleges in my country are advertising their .edu website in media.
.edu is for "Educational" which is a Generic Top Level Domain (gTLD) - Not associated with a country. However, registrants are limited to USA institutions.
If you go back up top and read the restrictions on who can own a .EDU TLD, it is only available to Accredited Institutions in the United States.
There are alot of sketchy and nonsense sites on .mil and .gov out there. I can't remember them off the top of my head but I'll post in a bit when I find them. I remember reading that maricopa.gov(PR6) accepts advertising now (maricopa.gov/Assessor/Ads/Advertising.aspx)/
I get a 404 when visiting that page. If you review the restrictions on the .GOV TLD, you'll see that advertisements are restricted to...
No Non-Government Advertisements A .gov domain may not be used to advertise for private individuals, firms, or corporations, or imply in any manner that the government endorses or favors any specific commercial product, commodity, or service.
As somebody who works in the government and is [in]directly responsible for our sites content, I can assure you that things that should NOT get published, do. While workflows are setup (as with any organization) to make sure that any content that goes live is approved/confirmed/legit/ etc., you would be surpised the stuff that I've seen pushed out at times.
Whatever content is pushed and may be questionable is nowhere near what is pushed in other TLD sectors. It is fractional.
Quick note - there are a LOT of "personal pages" on .edu sites where links are sold. I don't know if the algos can get down to this level, but it's probably an issue.
Let me point out that the TLD is just one signal of quality. If you read the information in the patent, you'll notice that there are hundreds of signals that are present. If links are being sold on .edu site in the United States, the institution itself is at fault and it will haunt them. They leave themselves open to the same crap that many deal with in other generic TLDs. This means that the institution TLD is open for poison and hopefully the person in charge of the website is monitoring the activity of the students. If not, then they are responsible for what may come their way.
Either way, the levels of abuse are fractional compared to other TLD sectors. Even if the student pages are not up to snuff, there are many other signals that a search engine can look at to determine quality and trust of the TLD.
To add to this, I would give .BIZ very little value and would tend to classify it as "untrusted".
You make a good point. And, since we are discussing .edu and .gov TLDs, we might want to start a new discussion on other TLDs that are of lesser quality. I can tell you that certain country TLDs are poison right out of the box...