We study hereditary properties of the class $\mathcal{A}$ defined by
Glasner-Monod of countable groups admitting an amenable, transitive and
faithful action. We consider mainly the case of amalgamated free products, and
we show in particular that the double of amenable groups and the amalgamated
free products of two amenable groups over a finite subgroup are contained in
$\mathcal{A}$.
We show that a class of cyclically pinched one-relator groups admits
amenable, faithful and transitive actions on infinite countable sets. This work
generalizes the results on such actions for doubles of free group on 2
generators.