Objects of age older than 100 years are antiques. Due to many conflicts in the past, these objects very possibly can be once stolen from their owners or looted from highly important institutions to certain nations.
And that is just the small tip of the iceberg, the hidden and even more unpleasant is the hidden part - laundering of funds that have been gained through illegal and dark actions, like human trafficking, modern day slavery and arms trade to support terrorists.
Would you want to purchase from a seller who is involved in supporting evil and dark actions?
Here, you can read an article by UNESCO on their international campaign.