Last time the party was in Waterdeep, and wanted to buy a crap-load of magic stuff, I quickly threw together the calculation below to determine if they found the items they were looking for. (I didn't want to wade thru tons of rules and role-play countless negotiations....they wanted to buy and sell a LOT of stuff.)
BUYING ITEMS:
Step #1: Take the GP value listed in the DMG for the item and get the square root of it using a calculator. The result is the % chance that the item will NOT be found in a week of searching for it. Rolling high will be good for the PCs.
Step #2: If the PCs want to try Gather Information to improve their chances, make them make the GI rolls at a -10. Take their best result and add it to the search roll. Note a minus GI roll can result in their failure chances going UP for that week.
Step #3: Roll d% every week until the item is found. On the week the PC succeeds in finding what he wants, randomly roll to see what day of the week it was. This will tell you exactly how long, in days, it took to find the item...if that is important to anyone.
This worked well as it resulted in percentages like the ones you see below, (not including #2 above).
50 GP item = 7% chance of not finding it in one week.
750 GP item = 27%
2000 GP item = 44%
5,000 GP item = 70%
8,000 GP item = 89%
10,000 GP item = 100% chance of not finding the item. It must be commissioned, or the Gather Info roll better kick in.
This quick calculation actually worked very smoothly and the players liked it. Of course I made it VERY clear to them that this method was only used because they happened to be in Waterdeep. Anywhere else and their chances would drop drastically. As it was, it was kinda fun making the rolls and then saying something like "No, Mr. Half-Orc, you didn't find the +2 greataxe you are looking for this week. Are you going to hang around Waterdeep longer and keep searching?" The PCs eventually just decided to just MOVE INTO Waterdeep, see below!
SELLING ITEMS:
The party, while they were there, also wanted to sell a crap-load of stuff. I was again forced to come up with something quickly since they of course wanted full DMG market price for their goods. Because it was Waterdeep, I allowed them to do this:
Step #1: To get the best price, instead of selling expensive items out of a fly-by-night wagon, the PCs better find a consignment shop. Or make a shop of their own, like my PCs decided to do. Building rental and all. They basically moved in, set up a storefront, and decided to make Waterdeep their base of operations. They also had a lot of non-magic military stuff to sell, so they promoted the store as an all-purpose adventuring shop. They called it "The Wayward Warrior" and soon adopted the name for the party itself, as they by no means were retiring to tend to a store forever. But I digress...
Step #2: Once the shop is in place, have them set whatever price they want for each individual magic item. Have them give this to you as a percentage of the item's market value. This will be the base chance that the item WON'T sell in a given week. Again, rolling high will be good for the PCs.
Step #3: Choose one PC to be the primary store merchant or manager and let that PC make a Diplomacy roll for the week. The result will be a bonus to the % roll below. (BTW, in our group this same PC was also the guy who got stuck minding the store while everyone else was off adventuring. The player didn't mind as he had another PC.) If your group used a consignment shop, use the shops owner's Diplomacy roll. Hey, the guy needs to earn his commission somehow.

To make things easy for myself, I applied the same bonus for all items being sold in a given week instead of rolling each item a different bonus. Again, they had a lot to sell.
Step #4: Make minor adjustments to #3 based on anything else the PCs propose like posted ads around town, or getting the party cleric's church involved in advertising clerical stuff, or what have you. But don't go crazy. Give them +5 thru +15 % tops. This will also be a bonus to the % roll below.
Step #5: Roll d% once a week until the item is sold. This has the effect that if the PCs want to be greedy they will need to wait for the item to sell at their asking price...unless they get lucky. Note that regardless of what the actual roll in step #5 was, the PC will only get the percentage they specified in #2 above. That was the price tag they put on the item and no one is going to give them more than what they asked.
And there you have it. Like I said, it was quick and dirty but they liked it, I liked it, and seemed fair at the time. YMMV.
