Yes i hope you can put it for sale - with modification - but you need to have some source code modified too - as because you are selling the idea behind it and not the actually word-word open source script... Hope this helps...
Thanks for such a quick response! It helps much. I'll modify the source and some upgrades of course, the look will be awesome, and unique. Hopefully I'll be fine.
That depends on the license and your motives. If the script is licensed under a permissive open source license(like the BSD and MIT licenses), you can modify it and sell it under your own terms. If the license is a copy-left license(like GPL, which is most likely the case), you can still sell it, but you have to distribute it under the original license. In either case, you still have to credit the original author according to the terms of the license.