Yea, they are just regular vendor NPCs that stand around in town. but the difference between a guild trader and a regular trader is a guild trader sells items that belongs to other players.
As you can see the guild "The Skooma Smugglers" own this stand. Think of it as a company renting this stand to sell their company's merchandise.
Any player can walk up to them and buy whatever items the guild's members are selling, from that specific guild trader.
Each town has a area where there's like 3-7 guild traders standing around each other.
For example, if our guild gets this stand that the skooma smugglers have, any player that interacts with the guild trader at this stand can see what items we have for sale and can buy them from us.
Our guild trader will have our guild name above the guild trader so others know who the guild trader belongs to.
Its similar to buying a store in real life. If people want to buy the items your selling, they have to go to your store location to get them.