As with many retro games, many of the copies floating around on ebay are from ex-video rental stores.  They are those pesky silver stickers that are 2 layers thick.  The have the companies logo on the top layer, and once you remove the top layer they leave a sticky residue that either says "VOID" or has little squares.  Pics are below of both layers.

top layer on a blue Color Dreams game

bottom layer on a blue Color Dreams game

If you have ever purchased any games with these, you know how annoying it is to get these stickers off.  Some of the seem nearly impossible to remove.  I have tried several different ways, and have finally found a good technique for removing them.  This method works very well so long as the sticker is NOT on the labels.  If the sticker is on the label, your best bet will be to just leave it cause it will probably destroy the label if it is removed.

All you will need is some Denatured Alcohol from Walmart, usually in the paint/hardware section.  A good sized container is only around $5.00 USD, and will last you for months.  Get some kind of old rag or towel and pour some Denatured Alcohol on it, get it pretty damp.  Gently, very gently rub the 2nd layer of the sticker with the towel.  If you scrub the sticker hard this doesn't work well for some reason.  Rubbing it gently is the key to success.  Once the sticker slowly gets saturated from the gently rubbing, it will slowly start to ball up and come right off.  The whole process should only take 2-3 mins tops.  I can't emphasize enough that barely rubbing gently is the key.  After removing 100's of this annoying little things, this is hands down the best method I have seen.  It used to take me 10-12 mins to get just one of these off.  Hopefully this little guide will save some fellow gamers some time and energy removing these annoying little stickers.  Take care!!
~~NGD