If you do your due diligence on eBay you can be fine. I sold my modified 5,1 on eBay (a decision I have regretted sometimes as I miss it) and as a Seller, the transaction was flawless.
If buying vie eBay I would strongly advise looking for sales where the original Apple box is offered as it protects the lower feet from damage in shipping.
For Craigslist/Gumtree, just bring a couple of friends with you on the pretense that the MP is heavy and they are there to help lug it around.