I have finally got my paypal system working using IPN and a reservation system to ensure that two people do not buy the same product.
The problem is, the reservations last for 10 mins on my application. If a user reserves the product the 'timer' starts, he clicks pay and ends up on paypals website. He then leaves the computer for 10 mins and comes back. He then buys it. By now though the reservation is up and it is possible that someone else could have already bought it.
Is it possible to send a time limit with the paypal IPN request to stop this happening?
As far as I know, you cannot specify that a transaction has to close within a certain amount of time with paypal.
However, I see two options you can use:
Offer a full refund if they purchase and the product is not available. You would check during the ipn notification of the purchase and if it is not available, then refund. You might be penalized by paypal if this happens a lot, but I don't have a lot of experience with offering refunds.
You can specify that the button needs to check inventory see "Step 2 – Tracking Inventory With Your Buy Now Button" in the standard payments guide. Once the product becomes sold out, you use the api to notify paypal the inventory is zero (if there is an api, you might have to use the website for this), and they should be able to notify the user there is zero inventory. I don't have experience with this either, but it seems more complicated than #1, and I would probably go with #1 if this didn't happen a lot. I think in the end you still end up having to issue a refund if the user goes through with the transaction even after seeing there is zero inventory, but I'm not sure about the wording in the docs.