I do not see any problem with the set up you describe. Just what size of bark are you using? If you are concerned about aeration of the mix I would add an inert as a component to the mix. This could be large sized perlite or pumice or even styrafoam chips (packing chips). Since you did not mention depth of your mix you may want to consider using a coarse grade of charcoal on the bottom blended with a coarse inert with a depth of 1.5 to 2 inches and set into one corner of the tank a "tube" that sits on top of the bottom fill and extends just above the surface of the soil surface-it can be hidden by a piece of bark or the sheet moss. This would provide additional venting. The "tube" could be anything like even a yogurt cup with the bottom cut off.

They are selling this alive and growing on ebay and I wondered if anyone has tried it or knows anything about it? I wanted something for a shady area as a ground cover.

It will likely turn brown as it dies. My guess is that "sheet moss" is any moss that is gathered from outdoors and could represent any number of species from any number of habitats.

Thanks for resurrecting this thread. It is very informative. The presence of moss growing on the surface of a potted plant may or may not be something to be concerned about. I think it depends on the kind of plant you are growing and the kind of culture you are providing the plant. Generally the moss must be well irrigated as well as having the necessary humidity and temperatures needed for it to survive. Most plants that do not require copious amount of aeration to the roots, such as orchids, may be at risk of root loss since the moss would tend to serve as an indicator of soil decomposition. A careful look at your plant roots should be the rule when growing with a dense moss growth. Even with orchids there are some that do quite well with the dense moss, Dendrobium cuthbertsonii for example. The best rule is to know the cultural needs of the plants you are growing and be able to make a determination that the presence of a dense moss growth is a plus or a minus in your cultivation.