I've sold a number of records on eBay and find that 99% of my customers are repeat customers that trust my grading system. Because of this, I will actually take a photo of the sealed record (to show that was how I received it), and then open up the record and give it a visual grade, indicating this in my auction. I find once you build trust with the customers, they will want you to open up the record to make sure it is in fact in NM condition. Besides, many buyers will want to know the info contained on the record itself that indicates whether it is an early or later pressing, who mastered it and where, etc.
As for buying records, I will never buy an unopened record unless it is a super deal. I've never come across outright fraud where the record is clearly used, but warping is extremely common with these (the shrink wrap actually shrinks over time, and exerts pressure on the edges of the record). I would always prefer to have a trusted seller grade the record than to leave it up to chance. In the event of a mis-grading by the seller, if you have a good relationship he/she will almost certainly give you a refund.