I have had a couple of poor experiences as well but they were minor purchases. Unfortunately I just purchased some wheels froma guy in California and I didn't pay attention to his feedback score. I have wanted the wheels for a while so I sprang for them and now it's crunch time.
I had to threaten him with the Paypal complaint and ebay complaint process and more negative feedback before he would even respond to my email. then he wanted me to call him from Toronto to talk on the phone? I advised put it in writing so I can capture it for the record.
Luckily, he shipped my wheels yesterday. Unlucky for him, he still gets negative feedback because he pissed around and didn't meet the obligation.
Lesson: Deal with 100% feedback and a good number of transactions. Check for recent history and even then, cross your fingers.
Chris
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