When you "buff out" a scratch, what you are doing is lower the surrounding paint surface to match the depth of the scratch. There is also the "collapse the side" theory, where, if you think of the scratch as a "v" groove in the paint, the top of the V can be pushed into the scratch to minimize the damage. You should be able to find links in the detailing section that can provide a lot more inofrmation on this subject.
But back to your problem / question...
If you have the opportunity, take the car to a professional detailer and let them tell you what you are dealing with before you purchase. Often times the type of damage you describe can be easily improved with the correct use of the right polishing product.
The answer to your question will depend on how deep the scratches are. If you buy the car and want to improve the look of the damaged area, start with a "swirl remover" if that doesn't work, try a "machine polish". If you still have noticable marks your probably headed to paint but a last effort would be a polishing compound.
And, of course, be careful you don't rub through the clear coat. Light pressure with the proper product will provide the best results.
