PayPal IPN / general PayPal application design

Question

So I'm making a website for used goods. That is, a place where people buy and sell used goods, and I'm accomplishing all of this thru transactions on PayPal.

Keeping in mind that this is a used goods website, which means that there is only one of each item, I have a question about PayPal's IPN. Is it reasonable to assume that if I haven't received an IPN from PayPal in over a day, that I won't receive one at all? That is, when I want to mark items as available, I want to obviously take into account whether someone has purchased them or not. But the only way I can know this is by PayPal's IPN service. I've heard rumors it can take hours, but can it take days for an IPN to come through? I don't want to keep an item unavailable for days and days just because someone added it to their cart and forgot to click 'checkout'.

Basically what my question is... how can I design my application so that it doesn't allow users to purchase the same item, yet have a turnover so that if a user keeps an item in his cart and forgets about it, another user can purchase it?

And the less conceptual question: does PayPal ever really take more than an hour or so to send an IPN?

Edit: I also realize that there's probably no time-efficient way to accomplish this without having some circumstances where 2 users buy the same item, but if that doesn't happen 99.9% of the time, I'm fine with delegating that to some sort of customer services part of my site. I just want some professional advice, since I am new to online marketplaces.

Answer 1

IPN Documentation here: https://cms.paypal.com/cms_content/US/en_US/files/developer/IPNGuide.pdf

I have my own web store, IPN responses are usually complete within one hour, I've yet to see one take longer then that and most responses are nearly instant.

You should not be decrementing quantity until you receive a response - this will solve your abandoned cart issue. If you are letting Paypal handle the quantity, I would imagine Paypal would not decrement the quantity until the payment is processed.