After numerous experiments I have discovered how microsoft track win8
devices in their store.
This information can be used to reset all trials and make microsoft
see your device as new - never seen before - without reinstalling windows.

To install something from windows store you need to enter your liveID.
Microsoft under your liveid keep track about everything you have ever installed,
license status - full,trial,expiration date.
If you move to another device and enter the same liveid - license sync occurs.
If you install a trial app , uninstall it, enter another liveid and
try to install it again - windows store says "you have installed this
app on this computer before" and prohibit installation. Installation
is possible if trial is not expired and you use the same liveid
you first used to acquire trial license on this device.
Furthermore, If you even reinstall windows and try to use previous liveid
store will prohibit installation.

Reasonable question arises. How microsoft track your device.
I know exactly how. It tracks MachineID GUID.
On clean windows there's no MachineID.
First time you install any application from windows store MachineID is created.
If you use you previous liveid microsoft query their database for hardwareid
presented by your windows store application.
If it finds something similar it return previous MachineID with all logical
consequences. Store will not allow you try and try the app endlessly using different LiveIDs
on the same machine.
MachineID association is supposed to be one time and forever.
It is never supposed to change.
Idea here NOT TO USE YOUR PREVIOUS LIVEID.
If no MachineID is associated to the device and you use clean liveid
microsoft has no way to associate your device with something from their
database. MS does not verify your IP. It also assumes there may be
many computers with the same hardwareid. The only piece of information
that help to track you is your LiveID.

Second question is. Is it possible to remove MachineID without reinstalling ?
The answer is YES.
MachineID is stored under registry key
HKEY_LOCAL_MACHINE\SYSTEM\WPA\39EEE4D3-6EBB-4C0A-8CBC-421AB72D114E-1

Once created this key cannot be deleted. It is protected by windows kernel.
But it is still posible to delete it.
You must boot from windows installation disk. Press shift+F10 and you will
get command prompt. Run regedit. Place cursor on HKEY_LOCAL_MACHINE node.
Map SYSTEM hive from your windows installation.
Common location is D:\windows\system32\config\system.
Remove
HKEY_LOCAL_MACHINE\<mapping node>\WPA\39EEE4D3-6EBB-4C0A-8CBC-421AB72D114E-1.
Then boot to your windows installation.
Go to windows store settings and sign out from your current liveid.
Enter there clean liveid. (I suppose you dont use LiveID for windows logon - its very bad idea !)
First time you install an app you'll get new MachineID.
And you will also be able to try all trial apps again.
Microsoft will give you trial licenses because it thinks you are new customer.

Format of stored machine ID :

<Machine>
  <Registration>
    <MachineID>12345678-cbd7-414b-c217-edb5660512ae</MachineID>
    <HardwareID>XAAAABMALgAAAAEAAwABAAEAAAABAAAAAQABAAEA+l6kPOeDJGbmt/7biAR26Hb5GDEAHA0AAgABAQACBQADAQAEAgAFAAAGAQAHAAAIBwAJAwAKAQALAAAMBwA=</HardwareID>
  </Registration>
  <Signature xmlns="http :// www .w3.org/2000/09/xmldsig#">
    <SignedInfo>
      <CanonicalizationMethod Algorithm="http :// www .w3.org/2001/10/xml-exc-c14n#" />
      <SignatureMethod Algorithm="http :// www .w3.org/2001/04/xmldsig-more#rsa-sha256" />
      <Reference URI="">
        <Transforms>
          <Transform Algorithm="http :// www .w3.org/2000/09/xmldsig#enveloped-signature" />
        </Transforms>
        <DigestMethod Algorithm="http :// www .w3.org/2001/04/xmlenc#sha256" />
        <DigestValue>92pU0KnXWLTC3Xnj+ka0hnLT2eYNcRD3auqW7jQ1u5o=</DigestValue>
      </Reference>
    </SignedInfo>
</Machine>

It is signed by microsoft and verified by WSService on its startup.
WSServiceCrk also removes signature checks from MachineID - it removes ALL signature checks from WSService.
You can tamper with MachineID. But if you change it manually and remove
WSServiceCrk - WSService will not be able to verify signature and may fail.
Better to delete it and request new ID. MS will sign it for you.

Also it is desired to delete
"C:\Users\user\AppData\Local\Packages\WinStore_cw5 n1h2txyewy\AC\Microsoft\Windows Store\*"
while Windows Store is not running.
It keeps there some local cache about trial expiration. 
