That would be a most unusual arrangement, vertical fire-tubes in a horizontal boiler. They'd need a smokebox along the the top of the shell; perhaps with the chimney in the centre, but from what you say this is not the case here.
Are they actually fire-tubes, or in fact a form of thermic syphon, so full of water in use? That is quite common on small horizontal boilers made for gas or liquid-fuel, with the burner heating the entire length of the shell.
As for value… I could not say but it would depend heavily on their condition, and evident signs of good workmanship would be a start.
It'd be worth asking if the shop also has any model engines etc, that look as if intended to accompany these boilers; or at least came from the same seller..