I am using GhostScript on a desktop application that I intend to sell online. The GhostScript library has an AFERO license (see here). I do not mind to make the source code of my application available to anyone who requests it. Therefore, can I license my application under the GNU Affero General Public License (GPL), even when if it will be commercialised?
Sure thing - none of the GPL licenses, including AGPL, forbids commercial activity.
See http://www.affero.org/oagf.html for more information.
Moreover, there are plenty of software with well established commercial ecosystem around it. MongoDB and RhodeCode what I recall from the top of my head.
Yes, but only if you license your product as AGPL as well. Just offering to provide source code to whoever asks is not enough.
That does not prevent you from asking money for the product, but it means your customers have more rights than you maybe like.
In particular, it would be perfectly fine for your first customer to put up your software for free on Github or anywhere else, also under the terms of the AGPL, so that nobody else will have to buy it anymore.
If your software is GPL licensed, you can charge: Any amount you want for the software itself. Reasonable cost for providing the source code. Nothing for the license.
How successful this is depends on the situation. If you wrote some customer specific software for a business, that business would have the right to ask you for the source code and hand the source code to everyone who wants it. But they would likely not do that, because that means giving the software to their competitors for free. So you can adapt that software for other customers and sell it to them as well. But if you have software that is of general interest, then there is a good chance that someone will give it away for free and hurt your business.
Of course if a business paid you for creating the software for them, and that payment pays your work, you are fine.