It's not uncommon for houses to be sold off-market (before actually advertised by an agent) and whether that's acceptable to you will depend on whether you believe you can do better or not. Have you spoken to the agent to ascertain what they were going to market the house for, and why (and whether you will save on fees if you accept this offer)? More importantly, did you get any valuations from other agents before signing the agreement with this one?
If you didn't, you can do a fair bit of research on value yourself as the Internet is awash with data these days. However, I'd suggest that you need to make sure you're happy with the valuation one way or another before accepting it.