Ebay does have good stuff but you have to know what you're buying... Definitely not the $200 stuff!! Many stores advertise on Ebay now so as to keep overhead costs low...
And yes offset and width will play a factor in your choice of wheels not just diameter... You could end up buying a 19" wheel made for a wide body 370zx or G37 and won't fit your car...
Try the search function on the top right of the page and it should pull up plenty of past threads on this topic...
Good luck
