How best to integrate a stand-alone webshop with eBay, using free software?

The idea here is the following: you have or want to have an eBay shop, and for some reason at the same time an own webshop (so you won’t pay the eBay fees when customers buy there). Then, of course, you want to save yourself the effort of double data entry, and manually syncing stock levels between your two shops, and also the uncomfortable necessity to use two software backends for order processing. What you need is a so-called “eBay integration” software, sometime also called “multichannel marketing” for eBay.

First: This is hardly possible with only free software at this time. Good news is, you can at least use a free software shopping cart solution as a basis. Also, we seek here for a ready-to-install solution, not a framework or software development service. And, that solution must be capable of internationalization of the UI.

The following shopping cart softwares have been evaluated as potential basis systems; and these seem to be all wide-spread free e-commerce solutions [source]:

1. Magento
2. osCommerce (not really considered, as osCommerce is well-known for its nightmarish source quality)
3. Zen Cart
4. CubeCart
5. VirtueMart (not considered, as it’s mainly for Joomla integration)
6. Ubercart
7. PrestaShop
8. OXID eShop
9. stand-alone solutions that come together with commercial eBay integration modules

Now the solutions that have been found. In order of recommendability, by my own subjective judgement:

1. M2E Pro. This is an eBay integration software for Magento. I have actually decided to go for this solution. Features and quality are good and professional, by my experience. The software is in active development as of 2012-05, with free updates. The price of 199 0 USD is very reasonable [source; as of 2010-09]. (Update: Starting in the second half of 2011, they now make the extension availabe for free as long as their partnership with eBay is running.) The only thing I would like to be different is that orders get immediately imported when something is sold on eBay, without the need to mark the article as “paid” on the eBay website first. (Update: As of 2012-02, this is fixed now.) Also, the usability has quite a “technical feel” (lots of templates to deal with). After all, it seems clearly the best option to me.
2. camiloo eBay module v2. While the version 1 of the camiloo eBay integration was only applicable for eBay UK sellers, this does no longer apply for version 2. However, version 2 is not ready yet (the beta program was announced for 2010-09, but seems to come later). Price is 199 GBP on release (approx. 250 EUR).
3. Innovicio Magento eBay integration. Good list of features. Price is approx. 400 GBP (500 EUR) once.
4. ProStores. The feature list and quality of the product seems to be really great. It also includes multiple UI languages.  Only about 20 USD a month if you have an eBay shop. However, I like buying software better than hosted solutions with a monthly fee.
5. Linnworks. This is a full “order processing” product that gets orders from eBay, Amazon, different own webshops (including Magento) and creates invoices, shipping labels, adjusts stock levels etc.. So it can do more than just integrate one webshop with eBay, and that makes it quite expensive. Also, there is a rumr that it is not intuitive to use …
6. Auction Lister Pro for Zen Cart. This seems to be the only free software solution at all for eBay integration. However, it is no longer under development since 2007. One could revive that, of course. See also the active ALP support group. There is also a variant for osCommerce.
7. Auctioninator. An eBay integration solution for Magento, and later, also for other webshop systems. It is first of all meant as a checkout solution for eBay customers, and will later (now?) also be able to post products from ones own webshop to eBay.
8. Techwave eCommerce. The smallest hosting plan that includes eBay integratio is Gold, for approx. 60 USD a month [source; as of 2010-09]. Again, I like software you buy and install on an own server better.
9. Kyozou, being a software as a service (SaaS) solution to pay for with a monthly fee. It can be integrated with ones own webshop, if you like.
10. AuctionBlox eMarketConnect. Offers eBay integration for osCommerce, CRE Loaded or ZenCart webshops. Price is 59 USD plus 1% transaction fee as of 2010-09.
11. AuctionBlox Storefront. A standalone webshop solution with eBay integration.
12. channeladvisor MarketPlaces. Offers eBay integration and integration with other marketplaces as well. Price is 44.95 USD per month.
13. There is a discussion about eBay integration with Ubercart, with a prototype site but no development since early 2009.
14. eBay integration for OXID eShop. This seems to be impossible to be done out of the box, but needs custom development based on the commercial eFire module.
15. Holbi eBay integration. They offer eBay integration for osCommerce, X-Cart, ZenCart, CubeCart and Magento. However, they charge 895 GBP for the thing (as of 2010-09).
16. RiteCart. This is also a software-as-a-service solution for 100 GBP a month for the multichannel capable variant.
17. NetSuite portal. It offers eBay integration [source], but it’s more a framework than a ready shop system.
18. M2E. This Magento extension is no longer for sale, it has been replaced by the more powerful M2E Pro.

For more information, see also this group on Magento / eBay integration.