If you're careful, I would say go for it. I have sold high end designer items plenty of times on eBay. Just remember that if you 'pay peanuts you'll get monkeys'!!
Definitely authenticate the item you are interested in at The Purse Forum. You can provide a link to the eBay auction page for the item, and they'll let you know if you need to ask the seller for additional pictures, or if they know already if it's authentic or fake.
Check the seller's feedback, and see what items they have recently sold (and bought) and what else they are currently selling. This all gives a good indication.
There are also a few reseller sites, try:
www.fashionbloodhound.com
www.highfashionsociety.co.uk
www.covetique.com
Check them out thoroughly though, as I've not used them myself before, just heard about them.
Other than that, I am a Bicester addict. Can't recommend it enough, you can get some FAB bargains. My sis bought a Carter Camera bag from Mulberry last week, down from £850 I think, and with my VIP membership (and her tax-free saving as she lives overseas) she paid about £270...
Best of luck, hope you get a wonderful new treat soon 