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.
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.
Maybe you can reach a specialized service? They will know better how to pump all the muck, also they have way better equipment. On this subject, I always ask for specialist help or qualified services. Last year, we had big problems with the garbage car. Due to all restrictions, in our neighborhood, they came once a week. You can't imagine what rodents problem I had! Happily, I found an online service with a mini bin from mobileskips.com.au. There I can order online a bin, whenever I need! In this way, I keep my trash storage clean, and I got rid of all the rodents!
I got a 2 inch trash pump with the goal of vacuuming the muck. On Amazon Prime it was about two hundred dollars, free delivery. Pump passing through solids up to three quarter inch max. We made a vacuum head to block gravel right at a half inch (I have lots of gravel for fish beds) Once you establish a solid flow (up to 120 gallons per minute), where any air in-leakage at the intake is totally defeating, you can go for nasty black muck and sunk leaves. It's a lot of water and nasty, needing a good place to go..
Like Fireishot said, have the seller fire it up and run water through it. Have him fire it up from being dry, not just run and shut off. How long it takes to prime is another indicator of how much wear it has - it should prime quickly.
You need to clean it regularly. Otherwise, it attracts mice and rats, and other rodents. Besides, everything starts with the insects, and then rodents come. I had one in my artificial pond at my house. Once, we left home for more than 3 weeks in a row. Obviously, the trash reservoir was filling up. When I came back, I noticed a lot of mice and rats eating from there. That looked disgusting. Luckily, I found this site — https://brightpestcontrol.com.au/pest-control-ballarat/. They did all the dirty job for me. I was afraid to come close to that pond because of all the mice (both live and dead).