That's a question rather difficult to clearly answer !
It's not a new thing that fantasy images were produced
on behalf of "uncertain" countries... but presently, printing
processes are far more easier to manage, and (almost) everybody
can now produce it's own stamps at home, on a legal basis, as
"personalized" stamps", provided you pay the tariff to the legal
postal authority of your country.
But that's not the exactly the point !
To start with ancient things, i.e. end of XIXth to beginning years
of the XXth century, there were inter alia, the following:
- The "octogonals" of Thessalia, mainly C.T.O., but possibly genuinely
"used on cover". A contested territory between Turkey and Greece.
- The "Myrditia" stamps, and also the "Chimarra" issue. A contested
territory between Greece and Albania.
- The "Neutral Moresnet" territory, at the tri-junction of Belgium, Germany,
and Netherlands, omitted in a former peace treaty.
- By the way, an other peace treaty omitted a formerly turkish island in the
middle of the Danube: Ada Kaleh, at the tri-junction of Hungary, Romania,
and Serbia. However, no Financial Genius had the idea of producing "stamps"
for this politically undecided territory ! So, hungarian stamps were used, and
cancelled at Orsova, the Hungarian town on the bank of Danube facing the
island of Ada Kaleh. N.B. Due to the dam built on the Danube, this island
nowadays no more exists, being entirely flooded.
- The Sedang stamps ! The 1888 issue has possibly been used, but obviously
not recognized by the then government of French Indochina! It was possibly
printed in Asia - Japan, or Malaya, or else -... Not sure.
The 1889 issue, printed in Paris, was an obvious fund-raiser, and never saw
any other horizon at that time than the one of the parisian gullible collectors !
- And "Turkestan", early "Armenia", "Azebaïdjan Occupation" etc... etc.
Then, for nowadays "productions", such as Hutt River, etc... Lundy Island:
Those ones seems to be what one can call "Local P.O." - Provided they are
when addressed, with real genuine stamps of the real country of origin, they
can easily "pass the test" of "circulated cover" !
Concerning the "TIBET" stamps of the Government in Exile: although the
Dalaï-Lama is welcome in India, not any country, not even India, officially
recognized his Government in Exile (fear of China's wrath and retalations).
They (Tibetans and the Dalaï-Lama) are merely admitted and accepted !
So, no hope for genuine "Tibetan Government in Exile Stamps" !
That said, collectors will obviously collect what they want to collect !
No problem !
But, a philatelist, genuinely interested in the postal history of,
for e.g. Albania, may surely show Myrditia or Chimarra "stamps",
at the BOC (well: back of the collection, not BOB !),provided the
"dubious status" is clearly stated and explained !
This is obviously part of history of what has been done,
not only warlike but also financially, to conquer a territory
by a little piece of paper !