Pim will reduce his prices for items that you are tring to by or raise the price on items you are tring to sell him . To get Pim to change his prices is quite a challange. First, select an item you would like to buy or sell . for this example the sapphire ring is pictured . Next, click on that item . the screen above would appear except the price would be 1000 Gold not 900 Gold . Finally click Don't buy. Repeat these three steps 8 times then Pim will say, "heheh, okay, you win...". At this point the price will be changed .  Here is a chart of the price changes:
Saphire Ring Wing Mirror
1000 -> 900 200 -> 180 500 -> 400
Ruby Necklace Golden Vase
1200 -> 1320 500 -> 550 2000 -> 2200