Good day,
Assuming your website will not sell to customers, the sellers (users of the site) will initiate a reverse process than the one that's regular in most online shops, and it could be very easily accomplished using codeigniter. The essential process would be like this: user mails product details to the site, the admin/staff looks at the email and decides if the product can be bought for the amount asked by the seller, he/they initiate the buying process by mailing the user with details on the next steps in order to acquire the product(s) - and this could be split into stages, with status messages for each product that's in the temporary inventory. The paypal payment could be the last stage, after the user confirmed all the steps for the exchange.
I have some more ideas regarding your project, but we should really discuss over skype or even in the site messenger, in order to iron out all the details.
Looking forward to your reply.
Kind regards,
Adrian Oporanu.