Hi,

can an investigator use location ability twice during one turn ? We thought you cant, and there is no exception listed in the rules, but what about when buying ally/items in shop/clues/skills?

The Science Building has the Dissection ability.
This ability allows an investigator to spend monster
trophies or gate trophies in exchange for Clue tokens.
An investigator at the Science Building may use this
ability instead of having an encounter there, and investigators
cannot use the ability if there is an open gate in
the location.

this is example from the rules and i guess it means you can buy more clues than once ... what about other locations ? Is it a little fraud in the rules ? You definetly cant heal to max health in hospital unless you pay 2 dollars ... but where is the line ?

The limit is in the phrase "instead of having an encounter here" - you only have (at most) one encounter in the Arkham Encounters phase, so you can only have at most one "instead of an encounter" instead of it.

This applies to all the "instead of having an encounter" locations

why is in the axample "monster trophies and gate throphies for clue tokens"?

Rather than saying "you can spend monster trophies or a gate trophy for clue tokens", you mean? Just slightly vague wording, I think. The text on the location on the board is clearer about what you can and can't do with the location.

(You can of course spend several gate trophies over multiple turns, so the example isn't exactly wrong)