No for non-potions/scrolls you will use the buying rules. Your character doesn't actually have to "buy it" it will just determine how lucky you were to obtain it.
So you will spend 100 gp
Make the Charisma(Persuasion) check (with a +1 per extra 100gp spent if you wish)
Check the chart to find out which items are available.
Then randomly determine their price by rarity.
This will give you about the same randomness that going through a dungeon might.
Also the 500gp will not carry over to your starting GP. So spend it all.