Hey. A quick search over the flea market files reveals that the game uses the generic spawners that spawn anything else at any of the festivals. This means that the furniture that shows up is predefined, which is a shame as I hoped it would have been based on tags, extending the value of that festival a lot.
There are around 180 spawner files responsible for spawning the furniture you can purchase, but you only would want to focus on the spawning objects. These are four examples that I could easily find, there could be more of them.
0x0000000000021621, # object_Festival_Spawners_FleaMarket_LightFloor
0x0000000000021620, # object_Festival_Spawners_FleaMarket_SitLiving
0x000000000002161D, # object_Festival_Spawners_FleaMarket_SitDining
0x000000000002151B, # object_Festival_Spawners_FleaMarket_Sofa
These contain lists of objects that will be spawned. The spawner itself is an object, with a spawning component, that selects a random spawning tuning file and creates the object to sell. To do this, you would need to clone a SpawnerTuning file from the flea market, change the ID inside of it to your object, and then add it to one of these lists. You can do it either via hard tuning overrides or via a python script.
I'm sorry if you're expecting a detailed explanation on how to do it, that would take some time to go over. You can do all of this in S4S, making a hard tuning override. It would conflict with any other mod that does this as well though, so remember that.
