Runehawk
this traders won't be spawn by default, you would need to place them with "/ven new" and remove "/ven del".
On the server we have one trader which stays at the spawn and one which visits cities(using other mods; vanilla traders + caravans are deleted), so idk if you would need them.
But if you still want, you can change items he buys/sells in "assets\vendor\entities\humanoid\cantrader.json"

I know this is intended for a server but would it work ok in a single player game/lan game with friends?

@Vronizos,  for 1.17 i'll add new version which copy vanila trader entity(with it's gui, texture, shape) but it will use coins from tradeomat. Trades will be set through enity's .json file, not with config file.

i have some issue with start this mod:

System.NullReferenceException: Object reference not set to an instance of an object

in vendor.src.vendor.Dispose()
in Vintagestory.Common.ModLoader.Dispose()
in Vintagestory.Server.ServerMain.EnterRunPhase(EnumServerRunPhase runPhase)
in Vintagestory.Server.ServerMain.Stop(String reason)
in _7zNxEbLnuy6Bq08TdMSIu9ZbrKf._ZHhrbmNLuSrtHTKBjZE8YscjKrG()
in System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
in System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
in System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
in System.Threading.ThreadHelper.ThreadStart()

@Kara, through the config file you can set categories and items/blocks vendor will buy. There is already list of default trades which you can use as example. But "maxAmount" of object vendor will buy is not depends on players and if 1st player sell to 0 others won't be able to sell anything before categories will be updated.
Time delta in which vendor update it's categories and "maxAmount" is also can be set in config and is 1 day by default.

if this is going to be a NPC that admins can configure.... this this will be amazing, for quests!!. <3