I've never seen or heard of any mechanics where any delay in opening treasure maps runs a risk of decreasing its value/payout. They don't lose value - its simply RNG, some maps give .70 nuggets, some give 2 Gold Bars. Average is about 1 Gold Bar. It is best to save your maps and open them when you have nothing else to do - like when your trader business is in production or you have moonshine in the works.
As for purchasing treasure maps from NPC treasure hunters, always decline their offer of $5 - They'll issue a counter offer of $2.50 instead usually after you decline $5 twice. If you approach them with a posse mate, they will offer it to the other player as well, so if you really want it, either take it by force for nothing or hope your friend declines lol