Prices for used shredders vary all over the spectrum, somewhat based on color, age, and 'heaviness'. I picked up a 6-ft Howse for $90 at an auction and have seen used green extremely heavy steel 6-ft models bring $1500.
I'd want to see it run on a tractor before buying it. Just to be certain the gearbox works and doesn't wiggle around on the deck. Look for leaks, fresh tool marks, gearbox grease level, chips cracks splits welds on deck and sides. If the price is right you can run it a long long time using grease-gun type tube grease in the gearbox and / or add a bottle of Motor Honey every couple years. When stopped reach underbeneath and feel the blades for nicks cracks splits and wiggle the blade up and down and check for free-rotation on blade bolt.
Concerning used, when you find one, contact the manufacturer and wait on them to send you the Owner's Manual for that machine. You'll need to furnish them the Model Number off the tag. If not present they can guess pretty well based on your description. You may need to send them a photo via email.
Some blades can be hard to find in some areas. If you have two different blade lengths----one's been cut off, ground off, or you have one or both incorrect blades. You need to start fresh with the OEM part number or you'll never get it right. Some blades will have a number stamped on them somewhere if not worn off.
There are literally hundreds of different bushhog blades--make certain you have the correct offset and direction of rotation and mounting hole size---in addition to length.
Locally, I pay about $50 for a set of two blades for a 6-ft Howse.
You might want to go back with new blade mounting bolts. Make sure the stumpjumper keyway slot is clean and make certain when reinstalling bolts the keyways line up.
Check local pawn shop for large sockets, extension, and break-over bar or buy a bushhog wrench to fit. Use a cheater pipe and penetrating oil. Lube the threads and keyway with thread sealant (pipe dope, not LocTite) when going back together.
After removing the nut the blade bolt will probably be tight. Put the nut back on just to top of bolt and use a piece of wood to punch the bolt loose.
Sharpen blades annually or as needed by removing and using a 4.5-in hand grinder.
Check gearbox and use 85-140-W gear oil or per Howse specs. Some of their gearboxes have a level indicating plug, some don't.
Tighten EVERY nut on the entire implement and go back with new NyLock nuts.
Blades can, do, and will bend and break. First time it happens you'll have a strong desire to be someplace else. Get out of PTO as fast as you can, go back to shop, clean seat, and install new blade set. Retighten all bolts, again.
Keep those U-joints greased. Replace missing zerks (one each end). Look carefully, may be sheared off.
Depending where you live and the stocking availability in your area you'll probably do better to buy a used cutter locally.
If this machine is new to you and you're not the original owner my suggestion would be to talk directly with the manufacturer and obtain the blade part number for that machine. And then buy locally if you can.
If you must, order your first set using that mfg part number.
Later, for future sets, you can shop for pricing using the KNOWN good sample blade.
Why you ask? I have seen used shredders with incorrect, torch cut, whittled, backwards, incorect rotation, upside-down, and everything in between blades. If you use what is now on the machine you truly don't know if it's correct for that use or not. I kid you not bushhog blades are magnets for errors. And believe it or not there are hundreds of possible blades to go wrong with.
As mentioned, some blades do have part numbers stamped into them. Most don't or are long gone due to abrasion and wear. And you still don't know if those are the correct blades for that machine.
If you've got a stump-jumper don't even bother trying to pull it. Locate the blade bolt and cut a hole in the deck to allow access. You'll need a 3/4-drive socket or a bushhog socket-wrench-on-a-stick blade wrench. My bolts are 1-7/8-inch. Yours will be different almost certainly. You MIGHT be able to fit a wrench over the lip of the tub-shape stump jumper. Others this forum report bending a wrench to gain access from the bottom.
If your first blade change go back with new special blade bolts, nuts, and lock washers. Clean the keyway on the blade carrier. Grease heavily when installing. Use thread lube on the bolt threads and tighten tight.
Change blades in pairs. Confirm installation per mfgs directions for orientation, direction of rotation, blade angle-up or -down. Spin by hand with blades straight to make sure not hitting deck anywhere. Support the unit if you're under it and DO NOT depend on tractor 3-point to hold it up. Might be easier to stand it up against a tree or stout fence post.
Grease gearbox and U-joints. Tighten 3-point lift pins and entire frame assembly.
Be prepared and keep in mind that blades can do and will bend and break---the first time it happens it'll scare you witless---turn off the PTO, clean the seat, and drive back to the house to replace both blades and retighten everything that shook loose.
Blades can be resharpened. See what new blades are like and duplicate on your blades when they need it. 4-1/2-inch hand angle grinder is just the ticket and can be done on the machine for a touch-up.
When using in tall heavy grass or light brush raise the tail as high as it will go and drive slow. If the cutter is set flat it'll bog easily.
Please post back your experiences so we may all learn.
