Is it normal to lose all my trade points when I leave an area? ill sell some things, but not buy back immediatly to try to save money, but when i try to go shopping later, my money is gone. is this a bug or is this a mechanic that encourages me to hold on to copper coins?

If you leave the area as in like back to the mapview. Then yes, your trade points will be there until you exit the location.

TP is pretty much shop keeper "momentarily remembering" that he owes you some change. 😅
If you walk out the shop – all of these burn up.
Coins are real money meanwhile, ones you can keep and hoard.

You can even Sleep in the Location, like Burning Keg after exhausting yourself to -100 Stamina, it'll keep the Trade Points until you leave / get kicked out.