With the Grand Market - the behavior, as it is currently, is a little confusing at times.
It seems like the behavior of +$ is "If you have enough to buy a Grand Market, play everything except coppers; otherwise, play everything." Or, at least, that's how it behaves in 90% of cases. However, in reality, that's not what's happening; what happens is that if you have enough to buy GM from just Gold and Silver, then play just the Golds and Silvers; otherwise, play Golds, Silvers, Coppers.
The corner case is when you would or might have enough to buy GM from Gold, Silver, and Kingdom Treasures, but not from Gold and Silver alone. That's the case that's confusing and where people get tripped up.
There are a bunch of possible solutions to this. One is to just leave it be and have people get confused and then eventually unconfused; as with most of these, they're not that big of a deal.
A second possible solution is to just have a better algorithm for whether or not to play the coppers - calculate whether, with kingdom treasures, it's POSSIBLE to get to $6, and if it's possible, don't autoplay coppers. This is the most convenient, but potentially takes nontrivial code, since it requires calculating possible values for Kingdom treasures, which have complex rules on them.
A third possible solution is to never autoplay coppers when the player has some special treasures in-hand. This will cause fewer accidental failures to buy Grand Market, at the expense of more clicks at other times. Whether this is an improvement or not is probably debatable.
A third possible solution is more buttons; to have, at the appropriate times, buttons for 'play everything' or 'play everything but coppers'. This one would be a terrible fit for isotropic, because the current design isn't really conducive to adding more buttons. It's a pretty tough UI design challenge to put in more buttons without things becoming cluttered and annoying; maybe for the official version.
(I'm actually really curious to see what the official version's UI ends up looking like. A lot of the things that are mentioned here CAN be addressed by more UI detail - for example, having "discard to Envoy" and "trash to Bishop" LOOK different, rather than just having different text - but that wouldn't fit at all with Isotropic's UI. I bet that a professional could design an interface that's as simple to use as Isotropic AND sophisticated enough to make those distinctions, but I certainly couldn't, and would have no constructive suggestions for how to do so.)