if you dont use the sell all feature it honestly isnt much different from the bl2 system, because the truth is, it didnt save you anytime at all.
In bl2 in order to use the bulk sell you had to open your menu, go through each piece of gear and select each one with the x. Now why would you do that when you could just use your down time at the next vendor to do the same thing- go through each piece of gear and just sell it, instead of putting an x on it.
Now in tps if you have gear you want to hold on to you do still have to enter the menu. However, there are three key advantages to the system now. A) there are going to be a lot less weapons that you wish to keep than those you wish to sell meaning, obviously, less selections to make. B) you only have to star a weapon once. with the bl2 system everytime you got gear, if you wanted to bulk sell you had to enter the menu and select all the weapons you wanted to get rid of. C) if you didnt pick up any weapons worth keeping since your last vendor visit, well happy day, you go to a vendor press one button and youre done.
As for you cobra, what you suggest is a falacy. You posit that every single person who uses the system will inevitably make a mistake and sell something they did not wish to.
The truth is, that is a possibility (and only a possibility, not a guarantee) in both systems… if you are careless. Before i back out i always do one quick scroll through the sell list, which takes about a second, and i check for any gear i may have forgot to star. I DO NOT leave that vender until ive looked one last time.
Now if you are selling each piece individually the likely hood of you accidently selling a legendary item is next to nothing. But if you are just spamming the confirmation button, selling everything cause you wanna get out of there it is possible to unintentionally sell some blue or purple you would otherwise have liked to keep.
Now i doubt you were being truelly serious with that statement, but my point remains. if you are careful and take precautions you can use both methods with confidence (well maybe not confidence but whatever, this post is already long enough haha)