In my opinion (I'm in the process of selling my startup) before spending money on lawyers there are a few things you can do, some already mentioned:
- Do your homework on M&A.
- Study the guys who have approached you. Based on your knowledge of the market try have an understanding of your value to them.
- Important: This is a SALES job. It does not matter how much your cost has been or even how much money you are making (well, it matters, but it's secondary). The key valuation factor is how much you are worth to them, i.e., what would cost them (resources and time) to build what you have built and take to the levels (revenue, pageviews, registered users) you have taken it.
- Which also means it's important to understand why they want to buy you: Your technology? User base? User engagement?
- If you have done sales before, or you're good at negotiating, meet with them and listen to them. You don't have to commit to anything, but it'll be good to hear where they are comfortable and where they are not. It's ok to initially give them ranges rather than concrete numbers to get the ball rolling - again it's sales and don't look at it from a hacker's perspective! While to an engineer a price range of, say, "10 to 20 Million" sounds too vague, in negotiation it's a good starting point. Do not limit yourself, just give them enough so that they start talking and giving you ideas.
- Whether to do it or not is a question that goes back to your goals and dreams. If you're looking for a job that pays the bills, you can gradually grow it, and you can be your own boss and work on your own schedule then why sell (I'm assuming you are making money). If you want to make a bigger chunk of money, say to buy a house, and at the same time have a good salary, then sell and become their employee.
- The employment agreement itself is also subject to negotiation. You can promise that you'll stay at least a year, so that you have the flexibility of getting out later. Or you can ask for special arrangement such as moving or not moving, keeping your site independently managed, etc. Again, think of what you would like to happen and talk to them. If they have approached you they will be willing to listen, and good negotiation will get you where you want to be.
- When you're ready to make promises hire a lawyer :-)
Good luck, and congratulations.