is buying can be a main verb; buying cannot, so
Ken is buying a laptop to take to school.
Fine. A main clause with a main verb, but
Buying clothes is much more fun than buying furniture.
"Buying" is in fact the subject (or rather "buying clothes" is). It cannot be the main verb because it has no other verb connected to it (like "is buying" in the first example).
The same goes for
I don't like flying in small planes.
In this case flying is a noun, as in "I don't like beans."
If loving your neighbor is difficult, think about how hard
loving your enemies might be.
Here loving may look at first like a verb, but it is a noun,
just as in the previous sentence.
[Can you figure out why neither
is nor might be can be the MAIN verb? If not, read on.]