Go to the machine and go to the tab machine. You can find all the characteristics of different machines at your disposal
Each machine requires a minimum level of training of workers to be used. If these do not have sufficient training to use a machine, they will not to operate. Therefore you can not make products. Look in the machine page for this information and if your training is too low, you will need therefore invest in training workers. To do this go to the "Employees" in the "Decisions" below.
Yes you have that possibility. But if you think it is too worn, it may be worthwhile to sell it and buy a new one, possibly more efficient.
Yes you have that possibility. However, it is necessary that at least one machine in your company. At the start of the game you can not sell your only machine.
The resale price is certainly lower than the purchase price. It takes into account the age of your machine and its wear.