you know this moment all too well. you are fairly deep into your campaign. your players have amassed a small armory of highly dangerous weapons apiece, are completely oblivious to the weight of their almighty silly endgame armor, have so much money that mundane expenses are irrelevant, and well, have no problems breaking the economy by selling the massive dragon's hoard that is their next haul. but you ask "how can i prevent this?"
i can't tell you a lie and say you can prevent it, but i can give you a handful of solutions to delay this inevitable outcome.
1. enforce Encumbrance in both the form of weight and the the dimensions of the items. many players hate being penalized for carrying their massive hauls, but sometimes, to reign common sense, you have to actually restrict the amount of bulk a character carries.
2. limit access to or even ban access to extradimensional storage devices. too often, we see "magic mart", and well even then, this bazaar of arcane goodies tends to have exactly what the players need, and what is the first thing purchased around the end of the prologue. more bag space, in other words, you get such a wonderfully overpowered thing in D20, and well, all its incarnations. a 5 lb bag that is literally used to carry every mundane tool you don't want to bear the burden of carrying. for 2,000 gold pieces, i can pay magic mart to deal with this burden of making an item to carry my loot for me. this actually kills the fun of prioritizing loot in a variety of roguelikes. because now, you can by the rules, carry everything you pick up without burden. better to ban magical bags and force players to buy a cart, purchase some donkeys, and rent a teamster and hire a team of guards.
3. use common sense. if your player characters slew 100 goblins and took no precautions to not damage their armor beyond salvage. please let them know that the leather armor is destroyed beyond repair, is goblin sized, and well, is probably really filthy. no Sane NPC is going to buy 100 suits of filthy, disease ridden leather armor they can't really use or repair, let alone try to pay you a reasonable amount for scraps that would have to be cleaned and refurbished and are well, a long way from achieving use. and really? who wants a goblin shortsword? everybody can already get a knife pretty easily. please don't think you can flood the economy with cheap knives and not expect it to have consequences. a smart leather worker or blacksmith has an immediate life to worry about too, sure, they could turn that goblin equipment into something workable for the other people in a matter of months, but their families don't have a matter of months to really wait.
4. please, don't use "magic mart", and apply common sense when restricting access to military grade equipment. whether you buy plate armor in a fantasy setting, which is restricted to nobility, assault rifles, which are limited to your nations military, or power armor, which is limited to the Federation's highest elite and established marine officers. you should realize, how much you want to restrict said equipment in the appropriate setting. it would be silly if everybody ran around with an M-16 or ran around in the highest grade federation armor, such things need restricted access. because they realistically aren't things that are available everywhere let alone accessible without permission of the appropriate factions or organizations in those settings.
5. enforce living costs, permit renewals and other realistic money sinks if you wish to delay your players from becoming the small armories they desire to become. a common example i suggest with power armor, is 6 laser weapon batteries, which i call fusion cells, 25 dollars and 1 pound each, can be used to power a suit of power armor for 12 hours of active duty. which is done to not only restrict ammo by providing another use for it, but also limit the availability of power armor in another way. effectively, people have to pay to renew their power armor and repair their gear. often for materials or whatever. i would say 25% of the group's haul is deducted to cover maintenance and lifestyle costs. maybe more if they have a bigger set of expenses to deal with.