Not that I'm encouraging doing this but I have sold items new but opened box, but I also clearly state as such. I have also sold many items used but were in like new condition (see heatware). For higher priced items I want to make sure the item works before I ship it out to prevent DOA and also to take pictures of the items, capture serial numbers, and etc for my records just incase the deal goes south.
I make the difference in that I take pictures of the entire process (example item Sealed, Item Opened, and Screenshots following for that product) and box it up when completed (<24hrs). If I had the product for personal use and decide to sell it, doesn't matter how long, it's used. I also list box was opened for pictures and testing in the same font as the item description, and don't try an hide it via other methods.
I agree there's many shady people out there on both buyer and seller sides, but take a look at the history of the seller before making a decision.
Personally in the beginning of buying/selling I had allot more trust in transactions but it only takes a few bad apples to quickly change trading habits. Example, worse case scenario if the buyer complains you shipped him a brick in the XFX 260GTX box, with the item sold as sealed, what evidence can you provide to prove otherwise? Also if the buyer complains the item never worked, reverses charges without even contacting you, what evidence can you provide?
I'm a very reasonable person and I find the only way I can deal with online trasactions is by taking these procedures to protect myself. I'm sorry if buyers don't want the terms I list, but at the same time not everyone is honest and everyone needs to be carefull on both sides.