If I was doing the buying I'd want the quality factory freshness that only comes with buying something brand new.
The two things I never buy on the used market are microphones and mixing boards, because there's probably a good reason why the current owners are parting with them and they both may have problems that won't show up until you've been using them a while. Guitars, drums and keyboards are all safe bets on the used market because their flaws are usually much more apparent.
The Soundcraft M series are tremendous boards for the money, but saying that is selling them short. They have good to excellent mic preamps, musical EQ and a clean signal path from stem to stern that no eBay Delta SR is guaranteed to come with. As far as the Revox goes, they made great tape decks and were never known for their mixers. If you bought one of those you might be letting yourself in for the nightmare of trying to find parts for a 20 year old mixer if anything ever went wrong.
That's my two cents worth.
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