The best method I know is by reading feedback skeptically.
Leaving negative feedback. Leaving soft positives like "Thank you for working with me on this". Any remark that says there was post-sale negotiation. You have to read between the lines.
And remember that this could also mean that the buyer is honest but has really bad taste in sellers. (Genuine one carrot (sic) diamonds from China @ 10 cents with free shipping? Yeah, right.)
You can set your Seller Preferences to Block some of the worst offenders. Bidders with non-payment strikes. Bidders without Paypal accounts. Bidders from countries with questionable import and postal services. Bidders with negative feedback, meaning less than one FB(-1) not just grumpy remarks.
You can also cancel any bid without actual explanation. A bidder who also sells your product might be an excellent repeat buyer but could also be vindictive about trying to kill the competition.
