I've had a few problems with some unscrupulous sellers on ebay, one moron tried to sell me some fake Abominus but he had applied paint to more accurately match the true colors so nobody could tell in the photos online but when it got to me it was clearly hand painted on, you could clearly see the brush strokes very poorly painted on and it still wreaked of fresh paint.
He actually had the nerve to claim it was still real haha so I demanded my money back and he gave it then I reported him for fraud...he got kicked off ebay several months later.
I had another seller try to say he mailed out my package just before the fourth of july holiday and gee it must have got lost in the mail...the auction had ended nearly a full month before that and I paid for shipping and sent a few emails with only one response from him ever of "oh I had an emergency" and that was all the communication I ever got.
I reminded him of the sellers shipping responsibility at the time was it had to be mailed out in ten days or less so he violated another policy...he states in his auctions that he buys priority mail boxes to "get a better box" charges those rates but then mails it out slow boat to china method.
I opened up a claim as he did not respond to any further emails...he never responded to the ebay claim and I got all of my money back...I left a negative and he retaliated and said "oopps my finger slipped to the negative" I replied in an email and said my middle finger seems to be doing the same

...I had the negative expunged from my account and he was soon after kicked off ebay.
There is a measure of trust with all members even ones you've done plenty of business with...this one seller I've dealt with many times I sent an email asking a totally reasonable question and they did not reply. the auction ended many days later with it not being sold and they re-listed it...I sent the question again thinking maybe they were busy or never got it but I never had those problems before but they did not respond again so now I don't feel comfortable doing business with a seller that won't respond to a simple question when they did not specify the condition in the auction.
Someone else will be happy to take my money and business.
There are some real jerks on ebay but you have to watch the terms of the auction and their descriptions...if they are constantly shouting in caps it's a turn off to me and ask questions always and check their feedback even if it says 100% sometimes there are things said that may be a warning of things to come.
Never go in to an auction where there is no pic or really fuzzy pics! there are still people trying to sell stuff without any pics at all.
Do always get insurance if offered, if it's not either ask to have it added or don't do business as it increases your rights and chances of successful monetary recovery if something goes wrong.
These are some bad cases yes but over all my ebay experiences have been mostly positive and there have been a lot of sellers who go the extra mile to help in certain situations.