According to the AutoIt license, selling your scripts is allowed.
The problem is that its difficult to protect your code like you can do it with real programming languages. This doesn't matter if you don't care that the source might be available to anyone who is willing to go trough the trouble of decompiling your scripts.
AutoIt is an interpreted language, meaning that all scripts are interpreted. But even despite of this weakness, you can still sell your scripts – it just wouldn't be worth the effort to protect your scripts, since they can be decompiled far to easily.
Quoted directly from the license:
This EULA grants you the following rights:
Installation and Use. You may install and use an unlimited number of copies of the SOFTWARE PRODUCT.
Reproduction and Distribution. You may reproduce and distribute an unlimited number of copies of the SOFTWARE PRODUCT either in whole or in part; each copy should include all copyright and trademark notices, and shall be accompanied by a copy of this EULA. Copies of the SOFTWARE PRODUCT may be distributed as a standalone product or included with your own product.
Commercial Use. You may use the SOFTWARE PRODUCT for commercial purposes. You may sell for profit and freely distribute scripts and/or compiled scripts that were created with the SOFTWARE PRODUCT.
Reverse engineering. You may not reverse engineer or disassemble the SOFTWARE PRODUCT or compiled scripts that were created with the SOFTWARE PRODUCT.
Selling AutoIt Scripts and Source Protection
Choosing AutoIt for important stuff is a bad idea – a real programming language is not much different – it may be a little harder to learn, but programs written in a compiled language are also much harder to reverse engineer. If you are using Visual Studio, GUI creation will even be easier than in AutoIt, thanks to their WYSIWYG editor for GUIs.