If you're thinking of buying ungraded cards and are worried about trimming, simply do this:
Ask the seller to measure out the card and confirm that it is the right size. If he/she doesn't respond, that may tell you all you need to know, but if you get a response telling you they measured it and it's of the correct size, you now have documentation of your concerns and their direct answers to you. So, if you buy the card and what they said turns out to be inaccurate (a lie
), you have proof to present to ebay to get your money back and more importantly (hopefully), any postage costs involved. It's not a 100% foolproof method, but it certainly gets you a long way there.