You can sell access to the update mechanism, repo, and domain name (and any trademarks you have registered), and any rights to relicense the software you have (not necessarily those of other contributers). For most intents and purposes this is selling the project. You can't rescind an open-source license retroactively, so the value of trying to buy such a project and turn it into a system with a proprietary license is probably low. Most users (and especially contributers) of open-source projects would see such an action as a breach of trust, so such an action is likely to result in a hostile fork even if the new 'owner' does not do anything actively malicious.