You're right: achèterais and achèterait sound exactly the same. In everyday spoken French, they sound like ach't'rai.

I don't know of any rule stating that the letters r and l are always silent, though.

Thanks chris only wondering because when listening  to the verb acheter the r is not pronounced as well as the r in Mars.

The -r in the infinitive of regular -er verbs, such as acheter, is never pronounced.