Ok here is what I have to say about online retailers, ebay, overseas retailers:
1) Places such as B&H, Adorama, they are reliable, they guarantee good service and products.
2) Ebay, other places, I am not so sure about.
But then you have to consider:
You live in one part of the world, you order a multi-thousand currency-of-your-choice equipment from somewhere on the other side of the world, it costs perhaps a small % of your product's price for shipping, insurance, etc. It takes a few days to perhaps a couple of weeks to deliver. It arrives and you may need to pay VAT or customs duties. Something goes wrong within the covered warranty period and you can get it replaced free, but you need to send it back to the place you ordered it from, takes a few days to reach them if you ship with priority express which can be very expensive, and from their side it takes the same amount of time as their original shipment for your replacement item to arrive. (Put it this way, while I was living in Oxford, I ordered something from ebay: cost me only 10GBP to get it delivered priority express (1 week) from China to UK, something went wrong with it and I went to the post office send it back for a free replacement. The post office was going to charge me nearly 50GBP for the same priority express option I selected when I made my order. The salary a Chinese postal worker receives vastly differs from the salary a British postal worker receives.)
That's only if you happen to be able to buy from a reputable retailer. What if the retailer isn't reliable? You might not even get what you ordered or if you're lucky you'll get it with a few missing accessories.
Secondly, remember that some brands of products might not even offer full global warranty.
I say, for stuff like camera bodies or lenses, buy locally or from an online dealer within the same country you live in. Another tip for lenses: first look in the secondhand market, many good ones are available, but even then buy only from a local dealer you can trust. You might even get some great deals. Try to buy locally or within a close geographical region because if something happens with your several hundred or few thousand $$$ purchase, you can quickly get the issue resolved without having to wait days to weeks and only being able to talk to the customer service over the phone or via email. It will most likely be quickly resolved as a short trip into town.
It is typical for a camera body to be upgraded more often than a lens, and camera bodies tend to get updated with new models every so often by the company. Lenses on the other hand will need to wait in terms of years before a new updated model is introduced (unless of course the model they recently released had serious flaws).
Small accessories like remotes or wireless triggers, you can buy on ebay since they are usually cheap and if something happens you aren't losing much in terms of financial investment.