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Run your fingernail (gently!) across the scratch - if you can barely feel it, then it's probably not going to be a problem. If it's a deep gouge, then chances are it will probably give you problems.
I have a few discs that have light 'scratches' on them, but you can't feel them using the above method, and they don't affect the disc playback at all.
If the scratch doesn't affect the disc's playback, then don't worry about it. Several of mine had very, very minor scratches when I bought them (particularly Buffy Season 2, because of the poor packaging) but they don't affect playback, so I'm not too worried about it.