I was, to use a British phrase 'gutted'. If I'd done the 'rude' thing and stopped the other assistant by explaining I was going to buy it, it would now be mine. But no, I'm far too nice for that
The fault lay with your vendor, who is a total e. They think they're being cute and clever by playing one customer off against another. That happens all the time here in the US. And when it does, I make it a policy to cut such vendors off at the knees. If they choose to frame the customer/vendor relationship in terms of competition rather than trust, it's best to oblige them by taking your money elsewhere.
And please don't fall for the ruse that all vendors behave this rude way and it's just the nature of today's world. A e is a e. There are very good, very honorable vendors out there. And it's a pleasure to give such enterprises one's loyal custom.
As unpleasant as these experiences are, they can often be the prelude to much better opportunities to come. Hopefully you'll find an online vendor who gives you much better service, selection, and price. Good luck!