You can usually tell which courier is being used by the postage price. If a seller has excluded Evri, the price will be higher, ie Royal Mail only will be £3.38, compared to £2.94 with both couriers enabled. (1-2kg parcel size).

Your seller obviously couldn't haven't cared less, but hopefully most sellers would be happy to answer a query about postage. If you were keen to buy an item, but only wanted it posted by Royal Mail, you could ask if the seller would be willing to temporarily exclude Evri. It is easy enough to switch the option off & on again.

Obviously, none of this is ideal, and it would be much easier to be able to see at a glance what courier is being used - although some sellers would list with Royal Mail postage, but actually send by another courier... so if you want to be on the safe side, it's best to check with the seller before buying.