If I would decide to purchase a flowerpot after Rosh Hashana, should I be concerned lest the seller relied on the Heter Mechira which I personally do not rely upon?
I assume you are referring to a flowerpot with already grown flowers. If it is only questionable and there is no readily available alternative, you may purchase the flowerpot. Decorative flowers do not have kedushat shviis as their usage is not by way of consumption. Hence, even forbidden work would not prohibit owning or enjoying the flowers. The only concern then would be supporting Shmittah violators.In that case, when it is doubtful and those who are lenient are relying on the opinion of their Rabbanim, this purchase would be permitted.