Its probably safest to ask as many questions as possible to expose any potential problems, and when it comes to payments you're going to be best off with an ACH or bank wire, because there you've got the theoretical possibility of a bank chargeback for your protection and they don't have to deal with credit card fraud or bum checks.
We've purchased several company's before. The best advice I can give is that if you don't trust the seller... then don't buy their clients.
Also, if the sale is over $5k or so, then you might want to invest in an attorney to handle the paperwork.
Never purchase anything from someone who is from a high-risk-of-fraud country. If you get screwed, then you have absolutely zero chance of getting your money back.
Consider using something like escrow.com.
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