1st of all, you need to know 18-55 and 50f1.8 will give you different 'user feeling'.
Have you try to take photo using your 18-55, fix at 50 mm focal length?
If you really like the focal length, you might sell away.
As suggest by CS TAN, why don't get 50 mm 1st. After all, 50f1.8 is quite cheap compare other lenses, so should fine for you, unless you really tight on budget, then I would suggest you keep using your zoom lens 1st
The auto-focus on the 50mm F1.8 is pretty slow in addition you may accidently break your auto-focus on the 50mm if you are not careful(had some friends who broke theirs), so it would be best to keep your 17-55mm.
Get the 50mm from OP, I got mine at about $115(need to bargain).
2nd hand ones are about $80.
Each has its own use and it can complement one another. Why wud u wan to sell?
That said, it also depends on ur shooting style.. But one thing for sure, there's limitation to 50mm (esp on APC cropped sensor), believe me, u will need ur 18-55mm :