There's a plug in entitled

Elgg Social Commerce Plugin in the repository.  The text mentions Paypal.

Same discussion covered here http://community.elgg.org/mod/groups/topicposts.php?topic=460303&group_guid=179063

@Amir, sure. You can integrate it. Have a look here http://webgalli.com/mod/featured_webgalli/paid.php or http://webgalli.com for some integration works

Regards
Team Webgalli

sorry i cannot quite provide a freebie demo for right now..

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but in may last year i was working on a particular project

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and for that project i had developed paypal and authorize.net integration
for a commercial $paid client.