weird tranny problem

I was looking at a 1991 B2600 for sale, guy wanted $600 obo, motor sounds and runs excellent, but the tranny is being weird. When the engine is off, it shifts into gears, but when its on then it wont go into any gears. otherwise its all good. Any ideas? it has some good parts on it, possibly could part it out, has good condition chrome parts on it (tail lights, wheel well thingies, and so on) and the engine.

might be slave cylinder or low on fluid

its most likely low on fluid,thats the exact same thing my mazda does,i just pump the clutch or trucker shift.as long as it gets back into 1st gear im good lol

can you force it if so then sounds to be the clutch or something that works it look under it on the pass side and have some one push the clucth if the slave cylinder moves then it is probly a broken or worn out pressher plate if it does not move then it might be the slave cylinder or master cylinder