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Could be the local PO released them early by mistake (which is the most likely case).
Yes. This happened quite often, as post office staff was often oblivious to the actual first day of issue of certain stamps. Fast forward to the present day and you will find that it is less likely to happen, as everything is barcoded and the USPS uses point of sale technology which is programmed so as to prohibit the sale of new stamps until the actual date of issue.
Although some of these early use covers are quite collectible, most of the recent ones don't really sell for much of a premium on the secondary market, as they are more of a curiosity than anything else.