question about selling and buying


Quick question about selling and buying, my players have a bunch of stuff to sell and they want to buy new gear too,so if they find a merchant they should do a diplomacy or intimidate check adding any other reputation bonus vs the profession ranks of the merchant or his sense motive check, and checking later the bargain or hard sell multipliers to set the final selling price, correct?

And how do i apply the inverse? lets say players have a bunch of crap, and want to sell it to get cash, i suppose they should do a tax check to set the price of the unknown item and them do the same check as they were merchants?
It really depends on where they are doing their buying and selling. Your buying understanding is correct.

If they are in a town that uses a merchant's guild, they will not be allowed to sell their merchandise to the general public. They will need to find a fence (see page 18 of Aquilonia).

You might check out the rules for fences in Hyboria's Fallen (page 59). Basically, the characters will get anywhere from 5%-50% of the item's value, depending on how interested the fence is in the item. Generic items tend to hold little interest.
Usually I will allow my players to sell the loot they find for half its value, no questions asked just to speed up game play. But this assumes they find someone interested in buying it. If they do, such as selling armour to an armoursmith or armour store, they get the 50%, otherwise, they will get less.

They can use Bluff to convince the merchant the item is worth more, opposed by the merchant's Sense Motive check (I usually give them substantial bonuses from synergies in their profession). They can also use Diplomacy to improve the merchant's friendliness towards them, but this usually results in smaller gains. Finally, if the group actually buys something from the merchant as well, he will usually cut a deal to give them a good price.

My group usually only tries to barter really valuable items such as gems and artwork. Anything less than 500 sp is generally just sold for the default 50% or less.
In the Shadizar: City of Wickedness there are also rules for this. It's a diplomacy check and the amount that you beat the target buy influences the percentage of value you'll get. If you fail you can try but you get a -1 modifier to your circumstance roll. After 5 or 6 tries the sale is over, you and the merchant are tired or just can't agree on the exchange.