What can be an adequate software basis for a sales platform with commission for the operator?

In this case, the requirements are like this:

• Features that allows trading of goods for fixed prices.
• Feature that allows the site operator to get a commission.
• Feature that allows people to have their own shops in the portal.
• Integration of payment systems.
• Running on Linux based web servers (as Linux hosting is cheaper and more common).
• Good internal software quality to facilitate maintenance and extension.
• Access to the source code for extending the system.
• Ideally free software, but not necessarily.

The following classes of software applications seem to be appropriate as a basis:

• online auction software (where only the “buy immediately” functionality would be enabled)
• online shop portals (offering shop-in-shop solutions)
• online shops (which can be extended to offer shop-in-shop solutions; but this is quite some effort)

Choosing auction software seems to be the way that needs the leas customization effort. The following auction software products could deserve a closer look; and it seems that these are all that is there and fits the criteria, currently …

In order of recommendability, the most adequate one first:

1. iLance with Stores add-on. The price would be 497 USD for iLance Starter [source] and 199 USD for the Stores module [source]. Both seems quite reasonable. The software is written in PHP and comes with full access to the source code. The code quality is unknown so far. But the Stores add-on demo looks good. Especially valuable for a site that is not just about selling is that there are more add-ons for discussion, knowledge base and moderation [source]. Last release is from 2010-05 so far, so the software is still in active development. Some people complained about iLance support back in 2007 [source], but so far not about their product quality.
2. PHP Pro Bid. The software includes stores right away, see this stores demo. Pricing is 135 EUR [source]. Source quality is unknown so far. Additional functionality is available in the module store (12 modules as of 2010-09). The last version is 6.06 from 2009-10 so far (as of 2010-09).
3. AJ Auction Pro. Offers stores integration, and is written in PHP. Version 3.0 is from 2009-03-17, and the latest version is 3.1 as of 2010-09. So the software seems to not be overly active currently.
4. WeBid. This seems to be the most active and most recommendable open source auction project currently. It is written in PHP.
5. VisualAuction. A mid-range auction software, for higher traffic than the other packages recommended here, probably quite professional. Also, the customer is allowed to modify the source for his own use. However, the last update (to version 9.0) was in 2007-12.
6. EveryAuction and derivatives. While EveryAuction itself is obsolete since 2002, there are highly improved derivatives. These however probably suffer from being based on very old code, and on the “dying” language Perl, and are available and sometimes not available under some “strange” licencing conditions. The source code is accessible though (as the stuff is written in Perl CGI). Source quality has not yet been checked.
7. enuuk auction platform. It offers a web store module (so, selling by fixed price) and a great many fee / commission options. Payment systems are also integrated. However, the software is written in PHP (which is quality software only in exceptional cases). A special advantage is full access to the source code when choosing to buy the non-obfuscated version [source]; that version however is 946 USD as of 2010-09-08 [source]. Due to the source quality problems in PHPAuction, the follow-up product enuuk is probably not recommendable either.
8. PHPAuction GPL 3.3. This is the last version of the free software (GPL) branch of PHPAuction, which later was renamed to enuuk auction platform (see above). The great thing is, it is free software. The not-so-great thing is, it is from 2008-08 and nobody develops it any further. The source quality has to be examined, but seems to be way too poor [source]. For this software, see also on auctioncode.com.
9. Drupal with e-commerce module. This was recommended here because the e-commerce module would have an optional auction module. The problem is, this e-commerce module does not allow user-created auctions [source]. Also, this solution (based on modules for modules) seems not scalable and flexible enough for a site that is a shop portal as a whole.
10. Auction Framework. The project is open source, but alpha stage and seems quite abandoned.
11. More open source auction software. But nothing was found that could be recommended here and is not yet contained here.