The problem with buying mailorder USA is inport duty, I have found it some what hap hazard but most annoying when posty knocks on your door and wants an extra tenner" unpaid duties". There are plenty of good home chanlers. See your Magazine loads of adverts.
as I said, there are loads of good ones, it depends on what you want to buy? Electronics and wet weather gear, I wouldn't buy from same place. Lots of recommendations if you narrow down the field a bit?
In the UK, for electronics JG Technologies are consistently good value (note also that their prices include courier delivery), or MES. For more general chandlery, there are lots of choices but Quay West Online seem to be growing well and have a volume-related discount offer, Seamark Nunn are very helpful, and Gael Force probably have the biggest range.
In the US, West Marine have an outstanding range (get hold of their catalogue!) and service to match. But they're not necessarily low-priced. Star Marine Depot have some good prices, but I don't know how good their shipping arrangements are. There's an electronics outfit called Dahhanes Marine which seems to exist mainly as an eBay store, but which has some very low prices and they ship internationally. If buying from the US, remember that shipping costs will be fairly high, and that import duty is charged on the value of both goods and shipping, and then VAT is charged on the whole lot.
SVB's prices for electronics seem considerably higher than UK prices - this is something I've noticed with a number of online chandlers in mainland Europe.