Should I buy this bike with a dent in the top tube? Is it safe? I'm considering buying this used steel-frame bike. But the top tube has a small dent (about one-third of the way from the saddle). Is this a problem? I plan to use it daily (~6-7mi) for commuting to work.

Here's the link: https://www.gumtree.com/p/bicycles/genesis-cdf-racer-/1361247559
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.