I recommend visiting www.freelancer.com or www.odesk.com - you'll need to find a "PHP Programmer". When writing the job description, explain that they need experience in coding a PayPal IPN listener and it would be great if they could show a working example they did. I assume of course that you're willing to pay a programmer. Expect to pay between $200 and $600 depending on how complex the project is and what country the programmer is from. Mind you, in most cases, you won't need an IPN listener unless you absolutely require 100% full automation on your website. I am actually a Web Developer myself but trying to avoid self promotion. If interested, google "Attrayant Web Studio", maybe we can help out based on budget etc.
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