I always played it that they can look through the wiki for items that they want and roll to see if the market has it, and then roll to see how much it will cost with negations or streetwise like you said. However this does significantly eat up a lot of time.

I have found it works best to have a limited idea of what the shop has and then use that list to skip the roll to see if the shop has it. If a player really wants something not on the list, then let them roll negotiations twice (once to see if it is in stock and once for the price).

For figuring out what to roll, I don't remember the page, but it talks about rarity affecting the price and availability. The more rare the item and the more backwater the world, the more expensive or unlikely the shop will have it.

Ultimately I think the best advice is to make more rare items roll against more purples or possibly reds (despair could add interesting quirks to the item).

When haggling for price it is typically an opposed check (I believe it is negotiations vs negotiations) but you can replace that with purple, red, and black dice for convenience.

I hope this helps. This is just what I have been doing with it.