My best advice for any piano you are considering buying is that you try it out before making the purchase. This is, of course, much easier to do at a local piano store, though not impossible if the online seller lives within a reasonable distance. Keep in mind that most online pianos do not include delivery, which can add $1000 or more to the price. You can tell how new a piano is by its serial number, a 6 or 7-digit number usually inside the piano on its plate, or carved or stuck on the back of the instrument. Then look it up in the Pierce Piano Atlas (most piano stores have a copy), or try to find it online at the manufacturer's Web site.
If you mean online like ebay etc. I dont recommened online purchasing. I went to get my first upright from a piano technician, he also sold/rented some pianos out. You could try renting if theres something like that where you live. And after a while you can purchase. You can also Find a piano store that does interest free, where you pay it off monthly interest free. Which that is what i am doing, its about over $100 per month, but thats paying off $7000 over 3 years. Its either that, or you can try buying a cheaper piano, but cheap usually is $3000 - $5000 which is still alot of money. I agree like someone mentioned, try it before you buy. Thats why online purchase isnt good. plus sometimes you have to arrange pickup yourself over ebay and there usually old pianos. Usually people selling there old piano to get a new grand or something