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Questions tagged non-market (× 6) are all about side-loading (× 17), some even carry this second tag additionally. Hence my proposal:
Yes: Let's merge the two, as it very likely covers the same questions/topics. The "Kindle Fire" mentioned by GAThrawn would be covered by amazon-appstore already, and other markets are covered by alternative-markets.
No: We want to keep non-market as "app source" next to alternative-markets, amazon-appstore, and google-play-store. Let's make this clear in the tag wiki.