The rule of thumb is a Vermeer baler can be run for 10-15 years and its worth at least 50% of what its sold new for. The baler needs 40 HP min and makes a bale up to 5' tall and 4' wide, about 1200 LBS in dry hay.
If I traded for the baler I would do the following.
1. Lace all the belts to the exact same length
2. Clean out the dirt in the belt tension springs
3. Track the belts by using the alignment adjustments built in the baler. The belts have some frayed edges on the twisted belts meaning they need to be tracked so the run true.
4. Advance the pickup timing 1/4-1/2" to make up for some wear.
5. Adjust the pickup teeth to within 3" of the square starter roller.
6. Adjust the wind guard so its travel is restricted to 1" below the top of the pickup teeth.
7. Repack the wheel bearings
8. Check and adjust the double twin tie and put new knives in it.
10. Check the toggle roller on the tailgate to make sure the allen head bolts are tight.
The condition of the baler is better than average if: The hay saver wheels do not show much wear so that is the first sign it has not baled great amounts of hay. The stripper bands,guide fingers, have most of their paint on them and seem to be in line. If they are in line that means nothing in the pickup is bent from hitting a stump or rock. The pickup is designed to bale 50,000 bales before the cam bearings need to be looked at. Sandy soil is harder on the pickup but if it was in much sandy soil the paint would be all gone off the stripper bands. The tongue should be straight if the baler has not been turned over. Check to see if the door on the leftside is true.
I hope this give you some things to look for.