You do have to "check" if the person making the offer is in a position to do so - that is done by relying on a representation by the company. By putting certain people in certain situations, the company represents that they are able to do certain things. If someone is on the front desk of a McDonalds, wearing a McDonalds uniform, in the view of a director of a McDonalds, then that director is representing that that person is able to take your order etc.
So the company has made some representation regarding the salesguy. The question is what that representation says as to the extent of the authority of that salesguy.
Caselaw is rather contradictory on the point.
There are 2 cases in England which illustrate the point one involved a "regional manager" of a bank, another involved something similar but slightly different - (can't remember exactly), let's call him a "branch manager". In one case the third party (i.e. you) succeeded as it was held that the bank represented that the manager could make representations as to the extent of his authority. In the other the third party failed - as no representation had been made.
There is a certain amount of academic debate about it all - for the reasons you have outlined, it is unfair on a third party. One academic believes that the agent has to be in a position of significant power in the company before he can make such representations. (Which would mean you'd lose). But you could argue that it is within the usual authority of someone in such a position as the salesguy was in. If it is within the usual authority of a salesguy to offer a refund then you are all set. (One other point is you say "he went off and came back and offered a refund - where did he go and to whom did he speak? If he spoke to a person who was higher up, that person was in an even stronger position to make a representation)
Basically the law is rather muddled in this area - I'm not sure anyone can give you a great answer.
Technically someone on the other side (either the salesguy or the company) is liable - if not the company, then the salesguy. If he wasn't able to make such an offer on behalf of the company he should not have made it.