When there is disagreement over a verbal contract there is never any proof positive one way or the other, it's just a matter of who is believed. Might go your way, might go hers, who knows?
On one hand you are very happy to tell us all what the law is regarding your suppliers and imho you are wrong in that, yet you seem entirely ignorant of other matters. Imho, and this is my last piece of advice on the subject, you might find that if you poke her and provoke her she can cost you a lot more than a pair of curtains, as 'simple contracts' which don't even specifically address things like debts and who gets paid for outstanding but already processed orders are in fact nothing but a complete liability.If you think about it also there will be some dispute about the 'customer's contract being with me' because you were not the owner of the 'business' at the time the contract for making the curtains was made. As you are so fond of pointing out, this is not a Ltd company, it is Mrs so and so trading as 'It's Curtains for You' or whatever it's called, sooooo, there could have been no contract with the company because there isn't one- it's just a trading name. The contract is with the previous owner which would mean that the deposit is hers, and the balance is hers I think. I think that will be her most compelling argument of quite a few she might employ that I can think of. It's not worth getting solicitor involved for such a small amount so I think you'll just have to roll with it.