I've bought maybe 10 or so photography related items on ebay, and had problems with two of them. The first one the seller took back with no problems, the second one I'm working on right now (it appears the seller is going to cooperate, I always hold my breath a little bit 'till it's all taken care of). I had one case a couple of years ago where a gearcase for an outboard engine was 10 years older than the seller claimed and wouldn't fit my engine. Seller was an ass and I had to file a complaint. It took ebay about 2 months to take care of it but I did get my money back, and the seller had to pay the return shipping as well.
As others have stated I try to stick with individuals who have over 100 or so transactions and at least 99.5%. I did however get a killer deal on my 70-200 from a brand new seller.
As far as import duty, I actually just looked that up today (I too am contemplating a lens purchase from overseas). It's not very much, only 2.3% (lens from Japan, I'm not sure if it differs depending on the country of origin). Supposedly it's supposed to be paid for items over $200, but I take it that it doesn't get collected all the time.