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I scrapped all the junk i had collected for years. If you are fixing equipment for profit and you need to warranty the work used parts can get bite you. Ask me how i know. i quit using any junkyard parts. If it needs a part it gets a new one. If it is NLA i don't spend unbillable hours trying to find one. At some point business is business. If you are fixing your own stuff or for a friend that is another thing.
Agreed, for the most part. Usually it's none moving parts. Never a bearing, belt or things of that nature. Seats, good tires etc etc.... With the exception of a good engine now and then. I sold & installed an engine for an elderly customer last year. Her fixed income wouldn't allow for a new engine or a new mower. I just serviced it for her last week. It still had good compression. The valves needed just a little adjustment. Cam & CR was still good. The engine came from some rich guy who mowed his place like 2 years, then hired a company to start mowing it. So his sat for like 3 years.
I warranty everything I do, for a month-ish.
My used equipment I fix and sell, I warranty what I do to them. When I take one in, I have a file on my computer labeled "new equipment," where I note what I do, the new parts I used, who I got it from and who I sell it to. And the dates. Those who buy it, knows what's warrantied. And he risk is on them.
With that said, on say like a riding mower, I check the engine compression and make sure it runs smoothly. New oil & filters & most of the time new plugs. Check the deck, bearings, pulleys, belts etc etc. Then check the drive line. Especially the belt and idlers. Tube all the bad tires. Sometimes even a good one that's starting to crack. Anything that doesn't look like it'll last a season, gets changed.
When you pick up a mower for $100 or less, and are going to sell it for $500, there's not reason to not fix everything wrong with it. But I don't want them coming back for something I just fixed. That makes me look bad.
That also being said, I've had some pretty good customers who understand they're getting used equipment.
Besides, I make more money on used equipment, than I do repairs.