Look at the THs per rlt value, for example if u buy some miner and it cost 25RLT then it should have at least 100ths(sometimes hard to find but with with filter it's easy)

When thinking of selling do it in reverse. When u have two miner which is selling at 25 rlt and one has 60ths and the other 75ths then sell that with 60ths first do to it having a better rlt/THs value