I'm thinking of getting a trash pump to move around bottom muck. ...
Be careful. Folks have had fish kills just starting up new aeration systems in old ponds with a lot of muck. If you use a trash pump to stir up the muck, you will be releasing a ton of noxious stuff all at once and could kill the pond.
Not a pro...just my 1 cent
You'll never know what ya can catch unless you wet a line!
What's the purpose of moving muck around? Are you going to pump it to one one side of the pond?
One thing to keep in mind is that the definition of good varies considerably between people, especially between buyers and sellers. $175 sounds to me like something's wrong, since the pump new is probably $1400-2000 and hoses are about $50-100 each for 50ft discharge or 15ft suction.
Worth checking out if you know what to look for, certainly! The seller may just be ignorant of its value, but think about what condition something would have to be in for you to be willing to sell it for <10% of your purchase price.
I've ruined a pump or 2, so a couple of things I'd check. First, make the seller sets up and runs water through the pump. It looks like it has a Honda, or a Honda clone, so I'd check the oil. These engines are built to run for hours, but the oil gets nasty PDQ. If the oil's low, or looks like black toothpaste, then it may not have had the regular maintenance it needed.
When pumping, the outlet hose should be fairly rigid. If you see multiple slow waves in the hose, then it may have a worn out pump. I would also cap the outlet and check for hose leaks. Either of those issues could wind up costing you more than the pump itself.
Thanks for the info, FireIsHot. I wouldn't have really known what to look for if shopping for a used pump (which is one thing that prompted me to buy one new one last summer) but this is good to keep in mind.