The writer probably chose that tense to indicate a repeated process. I would change "would" to "will".
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If you are buying goods FOB China, then you would need the services of a customs broker and freight forwarder.
I read the above sentence from an article. Why does the writer use a present continious tense (are buying) in it?
Can we use 'If you buy........'?
Yes, you can, but the progressive form suggests a process.
Can someone tell me what "FOB" stands for?
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.
Even though the Wikipedia link Mike posted states that FOB is only used in sea freight, I've seen it constantly used in trucking and air freight as well. Any time you are purchasing something large enough to incur substantial freight charges, the seller will always specify the FOB point. If the seller is paying all the shipping fees, he will quote the price "FOB delivered." If the buyer is expected to pay for the freight, the seller's price will be quoted as "FOB our plant" or "FOB Pittsburgh, PA" or from wherever the material is being shipped.