Customer managed to buy out of stock product

[FunnyBunnyHelena]

I sell unique products, only 1 in stock.

Now I have opened my shop for the storm of customers to attack my shop and grab the products they wanted.
In Zen-cart I used before the shop managed to sell only 1 product and then stop if there's more wanting to grab it at same time.

But here two managed to buy and PAY for same product. I had to go to paypal to refund the money and it is embarrassing to the customer it must be a big disappointment.

What can I do to avoid this dilemma?

I understand that sometimes people can put same product in the cart at same time, but I do not want people to actually be able to PAY if it has been sold already even if it is just 1 second ago.

Thanks for reading

EDIT:
The customer just told me that she saw that it was out of stock when she was in the checkout process but she continued anyway hoping that it was wrong.

Are there anyway to stop this? So that if it is out of stock in the checkout process it should stop the process?

[razaro]

Check in Back Office >> Preferences >> Products if "Allow ordering out-of-stock product" is set to No.
Also in back office on product page there is option "When out of stock".

[FunnyBunnyHelena]

Check in Back Office >> Preferences >> Products if "Allow ordering out-of-stock product" is set to No.
Also in back office on product page there is option "When out of stock".

The first: Already on No.

And on the product pages the products are on by default Deny.


But still she managed to continue and pay, I think I know why:

When it is out of stock you can't put it in your cart. But when you have it in your cart already you can checkout. This should not be so.

[goblinslave]

Actually the cart seems to monitor the stock levels right up to the customer shipping stage.

I was checking it out as I am building a website for a friend who also sells unique products.

Its when the customers enter the payment stage with the same product that it appears that the stock monitoring ceases.

[FunnyBunnyHelena]

Ah, I see. Thank you for your input. I hope this can be improved.