Is the airsoft Glock 18c good?

Sure, but it does depend on the manufacturer of the gun before anyone gives you a definitive yes or no. Spring gun? Gas? Who makes it?

If its spring, you can get hundreds of those on (see related links) if its gas, those are extremely rare and can be sold for about as much as you got it. However, only if its made by KWA will this happen. HFC makes one to its listed as "Darkhawk"