What I mean is... like when you buy a used car, there are certain things you look at first, and if they're not right, no deal.

Wear oold clothes, take a large piece of cardboard box, (corrugated is preferred) and a flashlight and some rags.
First, check engine and its compartment. Is the oil clean? (The oil can look clean, if the engine hasn't been run for some time, like a year, or even six months, from settling out.) Is the engine very dirty? Is the battery charged up enough to start it? Does it run? Are the tires good, not smooth? (Racing slicks do not apply!)
With engine not running, lay down on the large cardboard creeper, (Very useful when repairing or working on the lower extremities of the machine) and with the flashlight, check the condition of the belts and workings.
If you can test drive the tractor, do so. Try going up a grade, then down again. Do the brakes work? With this test, make sure you aren't pointing the tractor at your car, in case the brakes don't work! Does the machine seem to groan a lot, when driving it? Does it go readily up hills, etc., and back up satisfactorily! Groaning is a sign that a hydrostatic transmission might be on the "failure soon" trail.
Check the condition of the blades on the mower deck, to see if they are bent or chopped up from hitting rocks, and see if they turn without being too noisy. Or bent.

If you don't feel capable of doing any of the maintenance on the machine if you buy it, it might be cheaper for you to buy a new one to take advantage of the dealers services and care.
Hope this helps:
Rusty Jones, The Mower Man.

Good info. Thanks!

I'm pretty OK with routine maintenance and minor repairs (as long as I can find a "... for Dummies" book). I just want to try to avoid spending $1500 on a used tractor and finding out it needs a $1000 repair.

Thanks again.

Start the engine and let it run for 3 or 4 minutes. Then blip the throttle a few times. If this produces a lot of blue smoke the engine is probably worn. It may still mow grass O K but most of the engine's life has been used up and expensive repairs are around the corner.

It depends if you're looking at an old (more than 10 years) used tractor or a newer one. If a newer one, I'd look for general signs of abuse (scratches, dents, low oil, dirty oil, etc). That would indicate the owner beat the heck out of it. If it's an older tractor, just the sheer fact that it still exists means it wasn't completely abused, but I'd start looking for things that wear out under normal abuse. Look at the deck very closely, especially under the pulley covers, where moist clippings tend to collect and corrode the deck. Look for holes in the deck where it has rusted through from the bottom up. Cracks are bad but can be welded. If the bearings sound like a metal roller skate on cement they're shot. For an old tractor, if the deck spindles didn't have grease fittings, chances are they're toast. If the transmission is belt driven (most are), check the condition of the drive belt, which is usually hidden up in the frame and a real pain in the a$$ to replace. Deck belts are easy. Check for excessive play in the steering, which is mostly just annoying.
Check the condition of the oil, not too dirty or burnt-smelling. Pull out the air filter and see if it's even there, or how clean it's been kept. When you start it up (hopefully it's cold when you check it out), it should not blow out black smoke or the piston rings are bad. But if the gas is old, it might be overly smokey (and stink a little) as well. If it knocks (you'll know what I mean) the motor is worn out. If you see OHV on the head, it's a newer engine and easier to replace. If it's an older L-head engine, finding a replacement may be difficult.
If it's a hydrostatic, drive it around the yard for at least 5 minutes (offer to cut some grass) and make sure it doesn't get sluggish as it warms up. Check the operation of the PTO.
I wouldn't be overly concerned about blades or deck belts, as these are easy to replace. Tires are also easy to replace, but can be expensive. A single back tire for my John Deere GT225 just cost me $90.00 to replace when I slashed the sidewall.
Good luck. eBay and craigslist are great places to shop for used tractors.

If being sold by a private party ask why it's being sold.
If being sold at a dealer ask if there is a short Warranty period offered by them (30 to 90 days).
Alot of good things to check in previous posts.
Then you have to evaluate the answers.

Steve2ski Why am I selling it? cause I want too :) anyways thats the number one question, I sell rider's for my self and a friend that owns a mower repair shop, neither one of us like to answer the public's question's. It's part of the deal :( We pick them up, repair and sell em it's as simple as that, but if you tell the average buyer that, he will run like crazy ? No way can we give a guarantee, how do we know what happens to them once they leave our sight? I'm happy to let them mow my yard 3 times if they so desire :) Gives me a idea, I'll start offering that to prospective buyers :)

giventake,
Test mowing your yard 3 times is a good idea in place of a warranty, but I wouldn't take to time(I guess, you would need to put a price your time). I would just run, if I was looking for a mower to work for me, and not for me to work on it.
Good luck in your business, I just started my own business this year, after 43 yrs working. Alot of things to consider. It's fun, I do have a backup income, it's Uncle Sam's - SS.