hello ! n00b DM here !

as the title say how do I open a shop so that my adenturers can buy and sell their loot and equipment ?

does this exist, i dont know that why i'm here !

thanks in advance !

If you mean mechanically then you don’t. You do it with story and they adjust their character sheets for equipment and money.

If you mean in the game, you can do whatever you want.

So, say they want to buy/sell regular metal weapons and armor, you could have a regular blacksmith handle all of that in one town, but the next town over, all swords are exclusively bought and sold through the Swordmakers’ Guild, so they could do everything else at the blacksmith, but they could only buy and sell swords at the Swordmakers’ Guildhall. For leather weapons and armor they might need to go to a tanner (“leathersmith” type person). Or, if you don’t want to have to do multiple shopkeepers (as the DM), then you can decide that every town has a “Ye Olde Adventurers’ Goods Shoppe” and that every single one of them is just a magic door that all lead to the same shop with only 1 employee, but always at a different unique time “so that every group of adventurers always gets the utmost in customer service.” (Then, as the DM, you only have to do the one NPC.) It can really be whatever you want. They do recommend against shops to buy and sell Magic Items. There are good reasons both for and against them. Ultimately it’s up to every DM. 

Quote from IamSposta >>

If you mean mechanically then you don’t. You do it with story and they adjust their character sheets for equipment and money.

If you mean in the game, you can do whatever you want.

So, say they want to buy/sell regular metal weapons and armor, you could have a regular blacksmith handle all of that in one town, but the next town over, all swords are exclusively bought and sold through the Swordmakers’ Guild, so they could do everything else at the blacksmith, but they could only buy and sell swords at the Swordmakers’ Guildhall. For leather weapons and armor they might need to go to a tanner (“leathersmith” type person). Or, if you don’t want to have to do multiple shopkeepers (as the DM), then you can decide that every town has a “Ye Olde Adventurers’ Goods Shoppe” and that every single one of them is just a magic door that all lead to the same shop with only 1 employee, but always at a different unique time “so that every group of adventurers always gets the utmost in customer service.” (Then, as the DM, you only have to do the one NPC.) It can really be whatever you want. They do recommend against shops to buy and sell Magic Items. There are good reasons both for and against them. Ultimately it’s up to every DM. 

 awesome and thanks !

The equipment pages on dndbeyond show recommended prices for non-magical equipment. However magical equipment does not have a recommended price, and you are discouraged from having all magical equipment available for sale everywhere in your world. I choose select items to populate magic stores. I’ve hear Xanathar’s Guide to Everything has a magic item price list. I don’t own it.

I generally think about what shops would exist in what locations in my world. I ran a weapons and armor shop once in the upper-class area of a city. You could buy a lot of stuff there, but not leather armor. That was too low-grade for them. You’d have to go to the slums to buy that. But if you want to keep it simple, you could have a store in every major city that sells any standard non-magical equipment.

For magic items, try this: https://5emagic.shop/generate

It also can do mundane.

Quote from IamSposta >>

If you mean mechanically then you don’t. You do it with story and they adjust their character sheets for equipment and money.

If you mean in the game, you can do whatever you want.

So, say they want to buy/sell regular metal weapons and armor, you could have a regular blacksmith handle all of that in one town, but the next town over, all swords are exclusively bought and sold through the Swordmakers’ Guild, so they could do everything else at the blacksmith, but they could only buy and sell swords at the Swordmakers’ Guildhall. For leather weapons and armor they might need to go to a tanner (“leathersmith” type person). Or, if you don’t want to have to do multiple shopkeepers (as the DM), then you can decide that every town has a “Ye Olde Adventurers’ Goods Shoppe” and that every single one of them is just a magic door that all lead to the same shop with only 1 employee, but always at a different unique time “so that every group of adventurers always gets the utmost in customer service.” (Then, as the DM, you only have to do the one NPC.) It can really be whatever you want. They do recommend against shops to buy and sell Magic Items. There are good reasons both for and against them. Ultimately it’s up to every DM. 

 To add to this post. Set a GP  Limit for different town sizes and how much. Hamlet of a 100 people. Limit of it buying up to 10 GP twice. Different kingdoms may have different rules. No buying or selling great axes because Great King was murdered three reigns ago. Or you can do it off board before or after the session. Set limits doing that too. Loots sells at 10 % of equipment lists. Buying of magic items are limited to x GP. Poisons ETC are not for sell off board. 

I use the prices I. The PHB as a base price to purchase. Selling used stuff to a shop is at 50% new purchase.

Another thing I recommend is to figure out your economy and stick to it. If 1 CP is 10¢, 1 SP is $1, and 1 GP is $10, then stick to that (roughly) for everything. Otherwise people start to look at the comparative prices of beer to swords and scratching their heads. Be consistent and it’ll be easier.

There is also nothing wrong with changing prices depending on location. Prices in Barovia are high due to remote location and the PCs can't go elsewhere. Sometimes a shop may have an unlimited amount of something or only have one or none.

I had a vendor set up shop outside of a dungeon. He sold "gently used" adventuring equipment. The PCs didn't know what to make of it until one of them looted all the goblins and went to the guy to sell off a dozen shortswords and buy more torches.

Quote from Wysperra >>

There is also nothing wrong with changing prices depending on location. Prices in Barovia are high due to remote location and the PCs can't go elsewhere. Sometimes a shop may have an unlimited amount of something or only have one or none.

I had a vendor set up shop outside of a dungeon. He sold "gently used" adventuring equipment. The PCs didn't know what to make of it until one of them looted all the goblins and went to the guy to sell off a dozen shortswords and buy more torches.

 Oh, sure. Geography can excuse almost anything. But, if in the same town a beer is 1 sp but a Longsword is 2 sp, something has gone sideways somewhere somehow.

heres a template for a magic shop ive been working on. please consider leaving feedback so i can make it as good as possible.https://www.dmsguild.com/product/343543/meces-magical-merchandice?src=by_author_of_product