What really caused me to think on this more is simply because buying/selling took between 1-3 hours to complete, which is a pretty major amount of time to devote. To be fair, I could put everyone on the honor system for buying and selling goods, but this takes the ability to use appraise and barter down some prices which is always a nice part of the game. The biggest issue with taking so much time for buying/selling is that it takes away from the session and tends to cause distractions. My large campaign is in a situation where we only meet once or twice a month, and if we take up around a third of the time we have buying equipment, it really hurts the path of the adventure. The tricky part is that this element of the game can't be removed just for the sake of time because it really would take an element that is game-changing. Luckily, I have a couple solutions that have worked in the past for other DMs that I've played with, but hadn't really considered utilizing until now.
Authorized Spending Limit
One of my campaigns came across the issue where buying/selling took almost an entire session to play and as a result we found that just wasn't how we wanted to spend out time. As a result, our DM put in place what I'm calling the "Authorized Spending Limit" where you can sell anything at 75% of it's value on your own time (provided you were in town or would be shortly) and you could buy anything as long as it cost less than 2000 GP without the DM's interaction. Anything more would need to be rolled for to see if it exists and then could be purchased if it did. If a player wanted to roleplay out the purchase of an item, they would have to request it prior to the session so it can be addressed prior or in a small amount of time in the beginning. This method puts the control mostly in the player's hands because it opens up the possible treasures they could find while keeping big purchases under control.
Shopping List
Players can only buy what they submitted on their shopping list for the session. In my large group campaign, the buying/selling portion of the session took forever because of the "oh yeah, I need one of those too" mentalities. I would say that this method is pretty extreme because it does create a limitation for players because they have to really have a plan ahead mentality for this to work. this is a possible solution, but I would try other methods before this one.
Outside Session Purchases
If the players are all connected via social media or even if you meet in person more frequently than during game sessions, this may provide a solution. In this method players make a majority of their purchases outside of playing time. This allows players to barter if they want to and it also takes a majority of purchases out of play-time meaning if there are any stops for goods it should only be a couple minutes of bartering and that would be it. This provides the openness of a player actually interacting to get goods, but also makes sure that when you get to meet that most of the time is spent on adventuring and plot rather than on bartering for a +1 enchantment or a mundane bow.
As all my advice tends to be, trial and error is probably the best way to find what works for your group. The issue of Buying/Selling time generally isn't as big of an issue if you meet on a weekly basic (or even more frequently), but if your group meets less frequently, it is nice to get as much of the adventure into the limited time frame and leave purchases to primarily outside play and a little bit of in-game purchasing when needed.