I used to collect bits and pieces of trench art about 30+ years ago when nobody wanted it and it was considered worthless.
My only real interest was because of the ammo content initially.
In those days most of the pieces I picked up could be linked directly to an old soldier who had brought the items back from the war. Things like "my father brought that back " or “That belonged to my late husband, he was a soldier” so some of the stuff was contemporary at least but definitely not actually made in the trenches. More likely made to sell to soldiers by field workshop personnel behind the lines as a side line.
Some pieces, like the paper knife with a bullet handle must have been churned out in some volume because they crop up quite often. The same with the bullet crucifix. Best of all were the reworked artillery shells, some of them were beautiful.
After the war there wasn’t any real tourist trade in the area. Only family members visiting graves and they wern’t likely to buy souvenirs.
Even today the battlefields are not what you would call tourist traps with souvenir shops, there are a few places where you can buy bits today but you would have to know where to look.
Have I still got the pieces? No, my first wife hated them.