There are crunchy rules for this on the PDR, but for PFBB I would do the following house rule.
Equipment prices listed are in the GMG and HH are retail. A merchant will rarely pay full price for merchandise and will usually offer to buy for 50% of the retail price, so I would make that the default that a merchant would offer to buy. Obviously a merchant would want to sell something at 100%.
Next I would go with a Diplomacy check vs the starting attitude of the merchant (based on past relationship with the merchant). Success would alter the price +/- 25% based on if the character was selling or buying. A critical success would grant +/- 50%.
Failure would result in no price change, failure by 5 or more would worsen the merchant's disposition (PG 36 HH) for future negotiations.
It would probably depend on how many clerics were in town and how many temples there were. For healing, you could charge 25gp (50% of a cure light wounds potion).
For starting attitude, I would just go indifferent as most merchants are neither your friend or foe. If the merchant was a con artist then maybe Unfriendly. A Dwarf conducting business with an Elf would also be unfriendly or worse.
If the characters have a good reputation in the town, maybe the merchant would be friendly, or even helpful in the event the players did something heroic for the merchant like rescue them.
See pg 36 of the Hero's Handbook. There are 6 Diplomacy DCs for the 6 starting attitudes.
Player rolls a D20+Diplomacy skill vs the DC of the attitude.
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I used to allow a lot of haggling over prices in my game, over time I started to see this as detracting from the game and it began to annoy me more and more.
I think that 25% is too much of a swing in price and can alter the game economy too much if employed. Although I used to go nearly that far myself, I think it's more appropriate to cap it at 5 or 10%
currently I am sticking to prices in the book for purchases and 50% value for anything the PCs sell. This means we no longer spend 1 hour of a 5-6 hour session shopping and bartering, and allows for more exciting sessions overall.
If I was to allow haggling in the game again I think I would streamline it to be an opposed diplomacy check plus an opposed appraise check with each one giving plus or minus 5% to the price paid. If the player wanted to try this they would also be required to pay the higher price or accept the lower value if they lost the opposed checks, to add an element of risk to it. Of course they could always just opt to use the default prices instead of haggling in the first place, but not once they decided to try haggling.