What are Ancient Tablets for?

[Lemonpie]

Are they only for selling to traders?

[arjensmit79]

Pretty much yes. Except when your doing the map where you get archeologists digging up skelletons. There you get something for just having them in your storehouse.

[Franklin Hammer]

There are perks that give benefits based on the number you have. If you dont get those perks, then they're worth a decent sum of amber

[Silberfuchs]

I believe some glade events accept them as ressource as well, in place of other materials.

[Thror]

some orders require them as material also.

[Narandia]

Outside of the Crimson Orchard and certain orders, they are primarily just for selling, yes. Especially keep an eye on trade routes, those can be very lucrative offers.
Think there is also a deed for finishing a settlement with a certain number of them left in the storage, but obviously you only need to do that once (and probably will do so in the Crimson Orchard now).