Depending on the circs, I understand why people might want to go two's up to buy something.... (for advice/piece of mind/etc)

BUT.....

If I was going to buy something like an air rifle and was bringing someone else, then I'd definitely let the seller know this in advance out of courtesy.

If a buyer turned up with another person for no apparent reason and without explanation it would definately put me a bit more 'on edge' for want of a better turn of phrase.....