The Sort threads by "Last Post Time" should be set back to as default instead of the "Thread Start Time". It may not be that important in other forum categories, but it's crucial in the Sell and Buy section. People are forced to start new threads instead of bumping their own in order to gain attention, which leads to more clutter and waste of space. While it's not difficult to change the settings in the control panel, not all people here are tech savvy, thus sellers(buyers) may lose valuable clients(time).
So what you are asking is to change other user's sort order to benefit you? Sorry, not going to happen.
I asume their reason to change it is unstopable bumping... I don't know, but I agre with OP 100% taht previous order sort by last post was better. Now its really messed up. Not only for sellers but also for buyers. For sellers - their thread gets buried in 24 hours, nobody can find it. For buyers - don't get me started here, its a pain in the ass for me now.. Previously when you find a thread you can find many replies where you can get a good view of that service. Now - there are no threads with many replies, you don't have good view of how is that service rated etc etc.. no mouth to mouth etc..