The stamp or cancellation seldom make a difference. Out of a hundred old cards, maybe only two or three bear something significant or interesting. China's an exception to this rule.
All the philatelic items in my 28 boxes of cards (over 20,000) probably number less than a hundred. That's not to say I don't search postcard shows and antique-shop shoeboxes for hidden gems. I don't find many.
The provenance of the card is, in my opinion, mildly important. Cards to or from famous personalities, although selling one on
ebay has been known to result in a threatening letter from the estate lawyer asking HOW you happened to have the card. But it's easy to fake that sort of thing, just like it's easy to fake autographs.
Here's the most expensive card I ever sold:

It's Eugene Debs, frequent Socialist candidate for President in the period 1900-1920, standing on the front steps of the Atlanta Federal Penitentiary, just after his release, Christmas 1921. I put the opening bid at $9.95 thinking it might bring $20-25. It brought just over $300. Postcards are unpredictable.