Have you tried zen-cart ?
It's the same software that runs the seeedstudio shop.
I've just run a test install. It installs as easily as wordpress. Just create a mysql database and unzip to www folder.
It supports limiting the quantity of orders.
Thanks for your suggestions!
I would like to share some experiences with the community:
I tried both oscommerce and zen-cart. They are "flexible" but maintenance is much more difficult than a completely hosted solution. I don't know if they experience as much malware as wordpress and YaBB, but as they are well known to the hackers, keeping wordpress and YaBB clean is a taunting job. And if you start "customizing" the software, then forget about easy upgrades to new releases...
I am using WordPress for posting the assembly guides for my kits, I am using the SPAM protection plug-in and only registered members can post comments and the comments are moderated... anyways, hackers found a way to install a bunch of undesired advertisement and they also replaced the backup plug-in. I couldn't do any backup, so I had to manually copy all the posts, remove WordPress, reinstall a new version, and recreate all the postings... the problem was that the "new version" of WordPress changed the way the number the posts, and the post number was part of the permalink... so I had to change all the references to the assembly guides in the product listings, and the opportunity I lost for having the old permalinks in search engines that will find broken URLs... but you need to pay the price, as you cannot afford to show SPAM on your assembly guides, that are associated to a commercial venture.
I hope you find any value to these experiences...