Many factors affect my buying and selling of stocks and I'm going to split my answer into buying and selling. Hope this helps someone here!
Buying
The most important rule when I decide to buy stocks is my financial situation. My 1 rule is to never use leverage for the purchase of stocks. This is because if the markets turn against me, I stand to lose a sum larger than the amount than I invested.
Hence, once I saved up enough money for the purchase of stocks, I will look at the current pricing of the stocks and determine if the sector it's in is overbought/oversold, whether the stock is currently at its all time high, whether there's any significant news coming out for the stock that might potentially affect its stock price before purchasing. I also do not make single large purchases but save some purchasing power for if the prices go even lower, allowing me to take advantage of it.
Selling
When deciding when to sell, one general rule that I follow is to have an internal ROI that when reached, I will sell off my positions. To come to this internal ROI, I consider how the sector the stock is in is doing and potentially how it will do going forward, I consider any news of major impact for the stock and I consider if the stock has reached its all time high.
After selling, the funds are usually held in account to wait for further buying opportunities to grow my wealth.