Why so many...
 

...slips for sale & rent? I've followed slip availability since I splashed my new boat on the lake in '15. In late '14 I had to scramble to find a slip for rent and felt lucky to get one in Jan. '15.
Right now there are 16 for sale and 16 for rent on this site alone. This seems like a glut of slips now. Is it just more use of this site for advertising?

There is no glut. In fact they are quite short.
Most of those are probably outdated ads that have not been taken down
or people are are asking ridiculous money for trying to capitalize.