Most Chinese electronics companies are in Southern China, but you will have to be a veteran to know where specific specialties congregate. There is nothing in Guangzhou, at least not computer or multimedia electronic gadgets; but you find everything else in GZ. DongGuan is mostly textiles, heavy machinery and manufacturing equipment; I wouldn't trust a company based in DG to sell me a netbook. Avoid Hong Kong by all means, you will be paying premium for someone to take the 40 minute KCR train to ShenZhen.
Laptops are extremely expensive in China because of local demand. In fact, I exported IBM Thinkpads from U.S. to China at handsome profit. There are "shell" laptops that look and function like any other laptop, but they're just shells. The Chinese are excellent at making flash-memory based gadgets, but their harddisk based gadgets have plenty of catching up to do (we bought Chinese stuff and fitted them with harddisks in our houses in London and New York)
Most good Chinese companies can afford sales staff with excellent English communication skills. Though they're not always proficient, the professional sales stuff are polished and know their industry inside and out. Scammers, OTOH, usually have impeccable English but are not professionals; legit sales people will try to talk trash about a competition, while a scammer will tell you he can offer you his competition's products as well. Legit salesmen don't make decisions on the spot. If you tell them you want a container of this products, they will ask you if they can come back with an answer. Scammers have an enviable 'can do' attitude.
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Face, face, face! Don't lose face and don't cause someone else to lose face. Every word you say will be remembered and will be held against you; don't hint at a huge order unless you're going to place it. Your salesman is counting on a commission, and if he knows he has a big order coming he might just spend his few hundred last kuwais trying to impress you and host you for lavish dinners (and if you're not there, he will pre-invest that money in another client of his, treating him to expensive nights out.)
It's better to low-ball your ambitions and earn their respect when you place a huge order.
A legit company's salesman will probably be asking about your budget and MOQs right of the bat. That's how you can tell scammers, they're happy to sell you a "sample" and offer to deliver it DHL. If your legit company does free shipping, they will certainly don't wanna do DHL or FedEx or anything that expensive; if your margins are good enough pay for the shipping yourself.
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