When does telling neighbors what I think of their properties need a license?

Licensing is triggered by the activity, and the activity that triggers it in most states is negotiating, listing, showing, or bringing parties together for compensation. Sitting with someone and analyzing numbers on a property he found himself is generally further from that line than acting as his agent. Generally is doing real work in that sentence. The definitions differ state by state, some states sweep in anyone who advises on value for a fee, and a few have separate appraisal rules that bite the moment you put a number on what a property is worth. Your state's real estate commission publishes the statutory definition of brokerage activity, and reading it takes an hour. If it's close, ask a lawyer licensed in your state.

On the pricing data, roughly a dozen states are non-disclosure states, meaning sale prices are never recorded publicly. In those places everyone works from what the co-op and the closing attorneys and the local bank know, so you're not doing something unusual. What you owe a paying client is honesty about the source. Say "three people told me it went for around 140 and I couldn't verify it" rather than presenting it as recorded.

One thing to sort out before you take the couple hundred. If he buys it and it goes badly, what did he think he was paying for? Write two sentences describing what you'll deliver and what you're not doing, send it by email, and have him reply yes. That's a scope, and it's the whole difference between a consultation and a misunderstanding.