I think it's funny that Ruined Market is the best Ruin, yet the +Buy token is generally regarded as the worst token.
Well, first let's discuss why Ruined Village and Ruined Library are bad. Every action card has an inherent -1 action, -1 card associated with it: hence, a cantrip is a do-nothing action. An action card that only provides +1 action or +1 card is almost always worse than neutral. A Ruined Village doesn't increase your action count, and a Ruined Library doesn't increase your handsize. Whereas, the other Ruins offer a trade off: a poor effect for -1 action, -1 card. Usually the effect isn't worth it, but in rare cases it might be. Ruined Library has an additional penalty (when compared with the +card effect in general), in that terminal draw is less valuable than non-terminal draw.
So, why was Ruined Market rated higher than Abandoned Mine?
This is mostly a consequence of how people choose to evaluate the Ruins. If you evaluate the Ruins by which you're most likely to buy, Ruined Market comes out on top (then there's a 4-way tie for last, I guess). If you evaluate them by which you're least sad to receive (or most likely to play), Abandoned Mine is almost certainly better. If you evaluate them by which you'd prefer to trash last... it's a bit ambiguous.
This is because +buy has more variable value than coin. In most games, the +buy from Ruined Market is nearly worthless, because there are plenty of good sources of alternate gains, or there isn't much opportunity to buy more than one card. In a rare few games, +buy is so valuable that even Ruined Market might be worth playing/buying. In all games, an extra coin is meh. You can (almost) always buy Silver, which dominates Abandoned Mine.
There is another factor, I believe. People might overrate Ruined Market for the same reason that they overestimate how many kingdoms support an engine. I'm not entirely sure why this is. Perhaps people enjoy engines more, so those games are more salient. Perhaps people ban cards or skip kingdoms that aren't engine-friendly. Perhaps there is some signalling or groupthink involved: all the "cool" Dominion players talk about engines, so I'll think about everything (even, oddly, Ruins) in an engine context.