Everyone seems to agree with me, that they wouldn't pay £1300 up front for windows, both on here and in 'real life'.
Well not everyone, because I utterly
disagree.
I've just re-read the original post, and realised that I didn't read it properly the first time.
You've masked the issue by referring to these
special windows as if they were ordinary materials.
If the installer pays for these then he's effectively loaning you the cost of them until after the job is complete. This would be an utterly ridiculous thing to do for zero profit on the materials, so he's offered you the opportunity to buy direct from his supplier at his trade price.
Traders aren't banks.
When you engage a trader to do a job, you're hiring their expertise and time. The materials they use are merely what you'd have to pay for
up front if you were doing the job yourself. Hence, when traders provide materials that aren't special, then they do so with a markup that covers the cost of procuring the materials and of loaning the cost of them to the customer.
IMHO his approach is completely reasonable and businesslike, although his wife wasn't so professional in her conversation with you, but that doesn't change the underlying principles.
You're going elsewhere, but you'll end up paying over the odds for a company who loans you the cost of the windows and adds its own markup. If not, then you'll engage someone less capable who has to cut prices in order to get work.
Those are the simple economics of this situation, and your nose is out of joint simply because of your false perception of not being trusted.
People, perhaps not you, often forget that when the job is done the trader still has to do the following:
Produce an invoice.
Put it in an envelope and post it to you.
Bank your cheque (or cash) and pay the bank for processing it.
Replenish any stock that was used.
Pay suppliers.
Pay the bank for processing supplier payments.
Put fuel in the car/van.
Pay the bank for processing the payment for fuel.
Do the accounting, or pay someone to do it.
Pay the accountant.
Pay the bank for processing the accountant's payment.
Pay the taxman.
Pay the bank for processing the taxman's payment.
Pay the bank for the privilege of being able to pay them.
And, if there's anyone reading this who thinks that's all an insignificant amount of work and money, then why don't you offer to pay the trader a little something extra to cover the cost of doing it?