Either the seller doesn't know what they're talking about, or they are 'at it'.

Like all Seaforth Bns, the 10th wore MacKenzie, not Government tartan. Lots of pictures available online such as this one.

The kilt looks to be an Argyll's one from the first half of the 20th century, the capbadge and shoulder bar support this. As for the Royal Stewart Beanie...............