some softwares requires a key even for a demo : then you can encrypt a 30 days limits in that key you send to potential buyers
and you have the email of the potential buyer
when they buy the product, they receive a key with no time limit.
I remember reading somewhere in the forum someone saying that he used two or three hidden files in combination, but that was defeated by another member using a utility that tracks file system modifications.
Tried to locate the post unsuccessfully. I would love to use such an utility to verify a few things. I will appreciate suggestions.
I found this
http://www.macworld.com/article/1058081/software-utilities/findrecent.html
http://hints.macworld.com/article.php?story=20091223115101622
http://www.osxbook.com/software/fslogger/
search for fseventer… at least for os x
and the user of the forum was Dave if I remember correctly.
I have an app whose entire raison d’etre is the printing of files.
I have a non-time limited but save/print removed demo out there.
Still people think it is a 30 day trial (no idea why)
And to avoid buying, they literally leave the machine permanently turned on and use the screen display instead of a printout… 
I use internet serialisation and checking for the full program.
In the 6 years since I implemented that, Ive had maybe 7 users complain because they didnt have internet (despite ordering online!)
And a couple who it failed for
But non who didnt buy because of it… mainly because they dont know they need to register online until they get the full program.
[quote=266461:@Jean-Yves Pochez]search for fseventer… at least for os x
and the user of the forum was Dave if I remember correctly.[/quote]
Thank you. I intend to see how my competitors manage the 30 days stuff 
As I said above, I have been successfully practicing crippleware for some like 20 years. I also have been a member of Association Of Shareware Professional since the early nineties. But since then, the notion of trial-before-you-buy went through the motions. Since ButtonWare back in 1982 which was the full thing and would be called donationware today, many schemes have been used.
Today the market seems to have stabilized around a 30 days fully functional, nagware model. It has been adopted by the biggies such as Microsoft and Adobe. I do not think it is for small fish like me to stray from that model. Just like the MAS and its small free app+in App purchase.
So 30 days nagware I will go. Until if ever the model changes again.
Just a note. It is sometimes possible to defeat the 30-day trial by pushing forward the computer date by say 100 years, before installing. Then install and turn back the date to today. You now have 100 years and one month of trial. Make sure your software checks for such cases. Perhaps something like if the current date is smaller than the install date, then the trial period is over.
[quote=266461:@Jean-Yves Pochez]search for fseventer… at least for os x
and the user of the forum was Dave if I remember correctly.[/quote]
Fsventer has been discontinued according to MacUpdate, the developer site is down since July 2015, and apparently it does not work under El Capitan.
Still on MacUpdate they list some alternatives I will have a look at and report.
I also found a list of alternatives including for Linux and Windows at http://alternativeto.net/software/fseventer/
just found that : in terminal type :
sudo iosnoopheck ! fseventer user interface is was way better !
Super. It looks indeed very powerful.
OK. I have looked at the way my competition seems to work. It does put some hidden files in the system, but it also calls home (Thanks Little Snitch). Since it uses the Mac installer, it can put thing probably anywhere. What I found interesting is that in order to use iosnoop or fsLogger (fsventer site being completely gone and the program nowhere to be downloaded), system integrity protection must be disabled. But then the installer cannot start. So I could not really spot everything.
What is nice with their evaluation product is that it does not require any key or anything : install, run, no limitations, but at the end of the thirty days it stops working and asks for a code. Then a link goes to the site, with a button to register as a customer and purchase.
I do not feel like using the installer anyway. It is kind of rude on Mac. Now the most unintrusive seems to be to send home an encrypted key tied to the machine somehow, that will be recorded on the server with the start date. Upon launch the evaluation checks with the server if it can run, and if not, it presents a dialog that offers to enter a key or purchase it. I think I will point directly to a Paypal Payment page, and use IPN to update the key automatically, so the user can continue working immediately with his app.
I have noted Tim’s objection to calling home, so I plan on making the user aware the Internet is necessary to run the evaluation. In our age of ever more connected apps, that should no be considered a major sin.