Here is a listing of registrars:
[internic.net...]
Here is a site that shows Registrar Stats [registrarstats.com] (I am affiliated with this site indirectly). It shows what market share each registrar currently has. Look for registrars that have market share. The smaller the market share and the lower the registration fee the more susceptible they are to a shake up. All registrars pay the same $6.00 wholesale fee to Verisign Registry for .COM domains. So keep that in mind. You should be able to locate a good registrar for $15 or less now days.
Things to avoid: Ultra-Low prices because they offer little or no customer service. Look for things like phone numbers on the site and how easy it is to contact with them with questions after you own the domain. They all make it is easy to buy in the beginning as they can but contacting a real person later for support may be more difficult.
After you own the domain you are free to switch to any other registrar at any time. Feel free to do this if you are not getting the service you expect. Your new registrar will add an additional year to your domain expiration and charge you for that year. But you will keep all the time you have previously paid for.
Generally you put your money in escrow, he changes the admin email to your contact information, you transfer to another registrar to be sure he can change the details back using his reseller account, then the escrow releases the money to the seller.
This can vary a little, sometimes they will ask you to open an account with a registrar they are currently with, then they will push the domain into your account.
Unless you really really trust the person it is best to use an escrow service. Or to have the other person complete the transaction first. Then you know you are covered. Sometimes I complete the transaction first, then I usually send the person a contract and have them sign it and fax it back. I trust a person more if we have a signed contract.