Can one sell photos of cosplayers without permission if taken at a convention?

It would depend upon the rules put in place by the venue. Often times these conventions don't have regulations on taking pictures, so any pictures they take they are free to sell. No release or consent is needed.

The only time a release is needed is when the picture is being used in the form of advertisement, marketing, or as an endorsement. The reason photographers are always anxious to get a release is so that their pictures could be used for those purposes when they attempt to sell them on the secondary market.

If the venue has restrictions on picture taking (e.g. concerts and live performances), then those restrictions would take precedence.

edit: Often times as a courtesy, a photographer will get permission first, but it's not required.

edit#2: It has to do with the purpose/use of the images. Releases are needed when the image is used as advertising and/or endorsements. Consider that the paparazzi are able to take pictures of celebrities walking down the street and then can sell those pictures to whoever they want without needing to get permission from the celebrity. Your friend walking around the convention would be in the same situation as a celebrity. What he couldn't do is use those pictures in a costume catalog without getting the release.

you can release them for free.... you can't sell them. You need their consent if you are going to profit off of someone else.