Quote: "Can i just post my models to the TGC store, would i get the money for them?"
Yes and no. Before you can post models in TGC store, you have to fill out the paperwork and contracts to become an official artist. After that, you can upload your models to TGC store, and charge whatever you like for them.
Quote: "Also, whats bad about high poly models?"
When an engine builds a level, it has to account for every polygon in that level. The more polygons you have in your map, the longer it takes the engine to render those meshes. If you have a bunch models which, are all high-poly, then this may cause your game to lag. When modeling, it's best to keep your models as low-poly as possible to maintain a steady framerate.
Nothing better-to-do example: 
1 wooden board = 12 polygons. (low-poly)
1
detailed wooden board = 1,200 polygons. (high-poly)
Obviously, if you're going to make a static walkway with these planks, it would make more sense to use the board with 12 polygons. If you use 100 boards to make your walkway, using 100 of the high-poly boards would add
120,000 polygons to your level
However, using the low-poly boards would add only 1,200 polygons to your scene; and would look just as good as their high-poly counterparts in this instance.
You have to think ahead, because you have to consider the total number of polygons that will be in your level after ALL of your models (including characters, weapons, etc) are added. The more polygons a mesh has, is obviously the more detailed the model will appear in-game. But, in exchange for that added detail, you're sacrificing your frame rates in the process. Obviously this is a very vague instance, and you have to decide what models should be detailed, and which models don't need to be so much. How many models you'll have in your level, and so on. But as stated before; try to keep your models as low-poly as you can, while still making them look good quality.
Today, you can use normal mapping to take the detail from a high-poly mesh, and apply it to a low-poly model without adding those extra polys to your level. But...that's an entirely different rant.
Kravenwolf
