Is a bicycle frame with a dent in the top cross bar still safe to ride?

Sounds like you’re looking on craigslist. Maybe ask the person if you can take it to a bike shop to have it looked at before you seal the deal. If it does need to be repaired you could ask them for an estimate and ask the seller to take it off of the asking price. The other thing you want to know is how the dent got there. Was the person in an accident? If so, there could be a host of other problems with the bike that only a mechanic would be able to point out. You didn’t mention your overall knowledge of bikes, but by asking the question it sounds like you don’t have advanced mechanic skills. I would ask some more questions and get a second opinion at a shop if you plan to buy the bike. At the very least bring a friend along who knows a lot about bikes.

On a steel frame, those things can be cold-worked out. They put the frame in a wooden clamp and turn it 360°. It will mean a paint job. However, I’d just pass it by. You can find dozens of bikes for sale on eBay, most in good or perfect condition, at cheap prices; used bikes are a drug on the market. If you don’t see anything you like today, look tomorrow. There are listings all the time.