they have some leather seats (front and rear) on ebay from a sedan and I was wondering would they fit in my 03 zx5...the seats are from an older model sedan I think a 2000...is it real hard to replace the seats
Yes, they'll fit. But you don't want sedan front seats; they're different than ZX3/ZX5 front seats.
Notice how your ZX5 seats have nice big bolsters to hold you into the seat? The sedan seats don't have them. They will be considerably less comfortable and, particularly being leather, will not hold you in place nearly as well. The sedan seats also have this metal bar at the bottom that you'll feel all the time - not cool.
If you're going to the trouble of swapping seats anyway, get the ZX3/ZX5 seats instead.