I do mining in the early days when the difficulty is low that anyone setting up a mining rig could be profitable in just a year.
However, due the continuous difficulty, cost of electricity pump up, and taking consideration the maintenance cost if comes.
i came to the point that i need to sell off my existing rig and pause for a while. I even regret that decision when the price pump up.
But as an advice from an amateur miner, dont buy those used rig. settle for a new and latest rig that will surely make some profit.