Im pretty sure if a pump can pump water it can pump salt water. Unless its like some micro DC pump that has some crazy specific tolerances..
yes it can pump sea water....but for how long. salt water pumps have things like ceremic shafts. a pump that was not made for a SW aquarium would probably have a metal shaft to save cost. Salt+metal you do the math.
I love harbor freight, and flea markets, for those tools that I need to use once or twice. I wont spend big bucks on the #25 torx that I need to change a ford brake caliper so i go to HF. I have that $3 water pump. I needed it for a car trip where I wanted to refill water bottles. It worked great, but after a week the ball on the top came off.
so, buyer beware on the cheap stuff if you are planning on using it long term. But if you had a need for a week before the real one came in...
anything from HF will break quickly. there is a difference between throwing money away on a markup and saving money by buying garbage.