Should I buy this bike with a dent in the top tube? Is it safe?
A small dent in a steel frame should not be a problem. However, It's hard to see how big that dent is in the pictures provided. Damage to the top tube may indicate that something heavy fell on the bike. If the top tube is bent then the frame will be weakened.

Don't buy this bike without inspecting it bike personally. If the dent is small use it as leverage to negotiate the price down. Check for other damage too.