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Anthony N.Alternately you can scan with the lid open- this will give you a purer black background, since no light is hitting it.
If you gang scan multiple cards (4 graded ones fit on most scan beds) and have photoshop you can run the "crop and straighten photos" action to automatically separate them into 4 individual scans, all perfectly aligned.
Note that due to the holder, the above methods unfortunately do not work for SGC slabs. Those are a bit more labor intensive
If you select the entire bed of the scanner as your scan area you can do away with the prescan, saving an enormous amount of time. The excess will be cropped away in the above mentioned action.
I've had really good results with the Canon 4400f scanner- it's about $90 and does a good job with transparencies (slides) as well. Its the first scanner I've had since a Linotype $5k one about 9 years ago, and does a much better job faster.