i bought my cartier love bracelet on ebay and it's totally authentic! but, i did my research before we were ready to buy, so by the time i started looking on ebay, i knew what to look for.
not only that, i made certain that the seller offered a certificate of authenticity as well as a money back guarantee, especially considering it was $2,500.
he recently had it polished from cartier so i also had that to certify its authenticity as well as a brand new shiny bangle! it was only a year old at the time, and it looked brand new.
additionally, i made sure that the seller had perfect feedback, which he did. this valentine's day will be 2 years that i have had it and i have since brought it to cartier for authentication.
so, you can get authentic cartier items on ebay, but just know what you're looking for and what to look for (i.e. cartier stamp, serial code, etc. on the inside).
good luck!
oh, and the savings was over $1,000—so for us, it was worth it and i got a beautiful authentic bangle out of it!