I've bought from....
Dell (D-rebel kit), Costco (pocket SD camera + memory), B&H (70-300 IS, 420EX), Canoga Camera (a local retailor - several lenses & filters) and Ebay (10D, 20-35, 18-200 Tamaron, memory, bags, filters, 28-200 Canon, and more plus I sold a lot of surplus camera gear).
In light of some recent annoyances on Ebay I am shifting more purchasig to B&H. If I could buy from Costco I would in a heart beat given their liberal return policy. Dell is good too except to take advatage of their best coupon deals you have to fast and lucky. And as for the cheapest on-line dealers... I have heard of and seen investigative TV reports on the worst sorts. The TV investigation of one such shady internet dealer was quite revealing. It showed a place in NY which forbid anyone from walking in off the street and they had severe lowball prices... but they consistently engaged in high pressure tactics to force folks to buy accessories or items that would otherwise be bundles inside a normal package... like batteries abd chargers! Plus they had instances where they would charge a purchase and put an item on perpetual backorder. So - beware of the deal that is too good to be true.
The same goes for ebay. It no longer seems good enough to me to buy from someone with 100+ feedback with 98%+ positive. I had a seller with decent history (242 etries at 99%+) and a comforting listing refuse to honor their refund offer when the lense I won arrived damaged. Fortunately I was agile enough to get Paypal to force a refund ... but afterwards the seller sent annoying concluding emails - so I went and reexamined the original listing only to discover the seller had used fake ghost Ebay IDs to fake bids and drive up the price I originally paid! I turned him in to Ebay security ... and they suspended his 2nd account and canceled all his active listings which coincidentially had been using the fake ID to push the current bidding higher! AND - in his feedback history ALL of his sales listings with multiple bids had the fake ID putting bids all over the place! Plus, it seems there were 2 other IDs that looked suspicious. That left me with a sour sensation.
As a result I am NOT buying anything above about $100 on ebay again anytime soon. Just small stuff... like BIN goods... like filters and such.