If it's $1000 or less it's in rough shape. if it's $1000-$3000 it's in rough shape, but you might find a good one that is key turn ready.
Checklist for buying a boat
Look at where it's kept. Front yard covered good. Front yard under a tree bad. Is the persons house well kept. If so, the boat is probably well kept too.
Check the trailer. Is the rubber cracked, are the wheels rusted, push the boat around and see if it squeeks. Ask to roll the trailer a full rotation of the wheels. if your trailer won't get the boat to the water, you have an achor in your yard.
Look at the body of the boat. Any big scrapes, dents, cracks. Can be fixed, but do you really want to fix what you are buying?
Are there registration numbers on the hull? If not, you will need to send the paper work in. if there is, is it the current registration or the port of oragin numbers? Boats should have actual registration numbers. (Free to get the numbers) I think you have to have the numbers if the motor is over 10hp in the ocean, but I'm not sure about lakes. The numbers are really only used in case of trouble. Either you're in it, or you're causing it.

Get in the boat. Walk around and check the floor. Does it bulge, squeek. crack, flex? Not always a big deal. I've owned a boat where the floor was weak, but it held my weight enough to go fishing and bomb around lakes.
Turn the stearing wheel al the way one way. Check the hoses and cables. Are any pinching. Did the motor turn easily. Turn to the other way and check again. On some motors the prop will make the motor pull to one side or the other. This is normal, but it might also signify the lines need greese.
Is the stearing set up as a clothes line style? Line comes in from the left and right. I'm told this is illegal now and needs to be changed out. I don't know the facts on this though.
Check the seats. Are they ripped? Lift the seats, are they fastened down or just on boxes? Turn the seat all the way around if it does turn. Does it hit anything?
Electronics, Do the lights work, does the radio work if there is one. Look under the dash to see if the wiring is clean or a rats nets. both are fine if everything works. But you might want to loom up the wires yourself if you buy it.
Does it have all the lights needed? Get out of the boat and touch the lights on the boat. Some boats I looked at didn't even have real lights, they were just dead ones placed on top of the boat with no wires.
Ask to start the boat. Did the seller put a water boot on the engine or put the engine in water or just start it dry? Take the cover off and look at the engine. I looked at a few that were frankinsteined and ran away from them even though the price was dirt cheap.
Note: The engine is going to be LOUD, so seller may start it up and turn it off. Make sure it runs for at least 1 minute without stalling. if the boat is old it may take a couple of tries.
Is the propeller dented or pitted to hell? Props cost alot to replace.
Check the fuel hoses. Are they cracked or stained or pinched?
Bring a multimeter with you and test the battery. it should be 12 volts or more.
Does it come with everything like ski bars, extra seats, fishing stuff, kicker motor, down riggers? Are all of these things complete?
At the end of the day. if it floats and moves, it's a boat you can fish from. And the price is always negotiable. However you buy what you pay for. And with boats, the time of year is 90% of the price.
Check UsedVictoria and Craig's list for other boats of the same type/size for price comparison. And also look at boat parts to see how much it costs to fix one or how much you can get for parting out your boat when you are tired with it.
Boat stands for Bust Out Another Thousand. Keep that in mind. It's not a cheap hobbie.