By "Generic", the seller states the item is wrapped in a sleeve which is completely unrelated to the actual record. As such it can be (or has been) replaced with another (generic) sleeve anytime, by anyone.
Same applies for example to generic CD covers, without affecting the cd.
If a seller offers something like this, he will only have to grade the condition of the CD/LP; the "generic" sleeve can always be replaced (they can be purchased almost everywhere, usually in packs of 10 or more).
However, if the condition of the CD/LP is degraded from the original grading due to a sleeve in bad condition and/or poor shipping & handling, you should have the right to ask for a refund/new item.
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Chris from Belgium