Dealing with two people buying a unique item in an online shop at the same time.

Question

Hi there,

I'm creating a simple online shop with PHP integrated with PayPal that sells unique items. What I'm wondering is how other shops deal with multiple people attempting to go through the payment process with the same item.

This is my current draft strategy:

• Items have three status types: available, on hold & sold.
• As a user moves to the PayPal payment page it checks the status of all the items in the cart to ensure they're available. It also sets the item's status to "on hold" until they either come back after payment is confirmed or it times out (10 minutes? Not sure what this should be).

Is this standard practice or is there a more practical way I should be going about this?

Thanks in advance!

Answer 1

woot.com is notorious for this problem but their solution works well. After payment information is verified the user is brought to a page with a small amount of text saying something like "your order is in, we are verifying inventory".

It looks like the paypal API has a "RefundTransaction" message so something like that might not be possible. But the user experience could be awkward if your going to the paypal website then coming back to your site.

Answer 2

Have a look at Dell's UK outlet. When someone adds a system to their shopping basket it is held and not available to other customers. If it isn't purchased, the item is removed from the basket after 15mins of inactivity and is then available to other customers.

Answer 3

This is much like booking theatre tickets or similar online and yes the way you describe is generally the way it works. At some point the item is "reserved" in the system and either the customer completes the transaction or the item is released after some time for others to buy.

Of course at what point you reserve the item (when it's added to the cart, at the point you send them off to pay etc) is up to you. I would expect putting it in the cart would be the best choice as it makes it less likely someone will build up a basket of stuff only to find half of it is no longer available at the checkout.

Answer 4

This is a fairly common issue with fixed inventory systems such as venue, transport/airline tix etc.

I like the airline model where once you get the itinery you want and click select, you get a page with passenger info with a message saying, seats on hold and you now have xx(10/15) minutes to complete the purchase. Everything becomes explicit at that time. For other unique/one of a kind item, I'd think a message on any page, that the user clicks on, saying you have xx(mins) remaining to complete the purchase would be a big motivator for "on the edge" buyers !

Answer 5

I would say the first part of your strategy is correct - as you move to the payment page, flag all the products as 'on hold'

When the user has finished the payment, you will get a postback from Paypal which lets you know if the authorisation was successful or not (and possibly also allows you to check the CSC/CVV2 result), and at that point you have the option of either accepting the payment, or rejecting it.

On receipt of the postback you should also check whether the items are still on hold. If they have timed out you can reject the payment and display a message 'sorry - timeout exceeded' or somesuch.

This method also allows you to work out an ideal timeout period if you keep track of how often customers run into the timeout, so you can extend the timeout from (eg) 5 to 10 minutes if too many are timing out, or shorten it if none are timing out.