Buyer Beware:
Paying by 'cash'
Buying new double glazing and paying cash

Being serious again ... 
Make sure that you receive full agreed details of your order in writing and a legibly signed receipt on the double-glazing company’s headed notepaper for any money you part with.

Also, in my opinion, anything more than, say, a 10% deposit for replacement double glazing is truly excessive, particularly when you think that you will see nothing for your money for weeks or maybe in some cases - with the 'big boys' - for months.
Personally I do not take a deposit at all, and although this surprises many people, the answer is no, I haven't been seriously caught, to date.

Often the REAL reason a salesperson will try for a bigger deposit is because then he will get his commission straight away, instead of having to wait until you pay the company after the job has been completed.

My scam-exam test tells you far more about any individual estimate than my ‘buyer beware’ pages ever can because I have had to generalise very much on my website. Once you have put your estimate through the free test and had your results, you can then get an individual full five-page written report for a modest cost, and this should complete ALL the information you will need to be really clued in before you make any important decisions.

Further subject reading:
The best and worst ways to pay for new double glazing
Can I believe the double glazing adverts?
Double-glazing salespeople - can I believe them? 
PVC-U double glazing quality and strength - the ONE most important thing to look for