You can, indeed, sell the software
The GPL means you also must offer people [to whom you distribute your work] your modifications under the same license as you received permission to redistribute the original work - the GPL. So all it would take it one person to buy your stuff, and they can redistribute it after they get it from you.
The GPL absolutely doesn't preclude people making money from GPL software. It just precludes you from stopping people from redistributing your modifications to someone else's GPL'd code. Read Stallman's book
["Free as in Freedom"] for a cetechismic covering of this topic.