Don't want to write this by hand? GodCraft sets minecraft:trade_resupply for you with a switch.Get GodCraftOverview
minecraft:trade_resupply is an entity component: it goes in the components object of a custom mob, or in one of its component groups. Lets the entity refill its trades so used-up offers become available again.
Example
Inside the components object of BP/entities/<name>.json (values are the defaults or placeholders):
BP/entities/<name>.json
{
"minecraft:trade_resupply": {}
}