Mmm think about it from his POV.
Wednesday, he has a chance to go order the stuff for your roof. As these things vary, its fairly specific to your roof in type and quantity.
Stuff is delivered to his yard on Friday.
Sunday night, you text and cancel because the dogs leg fell off and needs 2K of surgery to fix and now you can't afford the roof sorting.
Muggins trader is now stuck with a stock of materials he can't use immediately, nor does he know when he can use it, and it needs to be stored, reducing his storage space for materials he can use/knows when he will use.
So now he's £1500 out of pocket, and storage space.
He only needs that to happen a couple of times in the same month and he is fucked, no more storage for goods/materials, but also no cashflow to purchase any either.
Does it make more sense now?
If you really can't stomach it, offer to buy the materials yourself with his instructions on where from/what, and store them at your own property til he can do the job. That way he can't run off with your money, and you can't leave him lumbered with materials he no longer needs.
He may not like this, you might fuck up and order the wrong thing or not enough, but sometimes people do it.