If you "assess -v" your pebbles, it might help you figure out what kind of shop might buy them.
But sometimes you just have to roll through your local market offering your pebbles (or any other item) to everyone in sight, hoping someone might take them eventually.
To add to this, try looking for crafter PCs, they might be able to tell you what kind of things they might be looking for, as well as which pebbles are not worth lugging around :)
Maybe you'll find a friend to work with!
if you drop pebble ~ is smiling with COLOUR eyes and a wonky nose. or arrange pebble ~ is painted with tiny eyes on. , your character can have extra friends* and there is almost no upper limit on how many
you do not need to feed them
* some may be intrinsically evil and are not able to be friends
When the pebbles start talking back as your own thoughts, then you have truly mastered the art of foraging alone in the wilderness.