Sure, if they have carts or the like, collect it all and liquidate it as soon as possible. Of course, protecting the NPCs and carts are a whole other concern. These are easy meat for wandering monsters in my games.
But if they don't have carts and NPCs (or mounts or whatever), and encumbrance is a real challenge, salvaging plate armor isn't actually as good as just going for gold since it's 50 gold pieces per pound. A suit of plate armor sold at half cost weighs in under 12 gp per pound. If you have to choose and nobody in the party benefits from just wearing the armor, better to go with the gold. (Or gems. Gems are even better in this regard.)
As well if I'm running a game where encumbrance matters and how much you can carry into the adventure location and how much you can carry out is part of the meaningful decisions the players must make, I would definitely not include bags of holding or anything like that in the game.
If the possibility is there. I see no reasons to prevent players from doing so. And since they do this at relatively low levels, it means that most wandering monsters will be low level too. So 5 or 6 enchmen can manage a random encounter or two.
Also, why would I prevent players from a perfectly valid strategy? Why would I prevent them from using their brains to get a bit more? It is not as if the amount of money they will get out of this will put the world's economy at risks. The local one, maybe. But at some point, preventing players from using any strategy is not an approach I would favor.
Also, why would they leave the gold and gems? It is not because you take what you can that you will leave out the rest.
And players can commission a wizard to create bags of holding which will cost them money which will in turn force them to adventure to get better gear to sell to get... It is a wheel that turns by itself.
That said. Not all campaigns will see that kind of play. This is best for small adventures approach like we had in the 70's, 80's and 90's. Big campaigns going from 1 to 15 are all the rage now and this strategy would not really work.
Currently, one group is in OotA and obviously do not do that strategy. The other group is exploring a "semi random" mega dungeon and they are doign that strategy all the way through. They are now 12th level (6 players strong) and their enchmen are on average level 6 to 9! So when they get back from a foray into the mega dungeon, they have people to help in healing, identifying magical items, repair stuff and to cook! These NPC are well paid and they make sure with Leomund's tiny hut that they stay safe and they get a nice share of the profits for a very small risk. When monsters start to reappear in earlier levels, they are the ones investigating. Sometimes, it means that the players take the role of their NPCs and start hunting where the main group have failed to find something. It is a lot of fun and it also change the pace once in a while.