I would say the general rule of thumb applies here, "If it sounds too good to be true, it usually is". There are assumable loans, but I've never heard of one where the buyer doesn't have to go through the qualifying process. And, part of that process is having your credit pulled.
I would advise you to have your own agent. This would be a real estate transaction, so it's a good idea for someone to be working in YOUR best interests.
They are perfectly legal and a tool investors use on a regular basis. Google the topic, and then please know these types of deals are usually for the professionals only. Once you steer away from traditional financing the doors open up to amazing financing opportunities, but it is also where the bad deals can be created and scams take place. However I can 100% tell you that successful investors know how to use these types of financing tools to the maximum.
A lot of us answered your other question on Trulia about buying a home with your fiance'. Depending on your goals a good Realtor who also works off the MLS can fit this bill for you. It just comes down to the interview.
We realtors are not the same, and we all have our strengths and business models we choose. Finding the right match for YOUR SPECIFIC goals is what should be running through your mind right now.
Happy House Hunting!