I have no clue what this is

I’m going to say it is what it says it is, a “Sales Trainer”.

My guess is someone from Standard Oil traveled around with one in the 50s/60s? going station to station showing slides teaching attendants/owners/franchisees the in’s and outs of sales. My guess is it was a rarity that stations had the equipment to play these “cartridges”, so this was the best option. I’ve never seen one or looked up anything about them, but you may find someone here who was actually trained with one! Wouldn’t think there’d be much value, $40? Not a clue.