@Edward Barrett
If you sell multiple anything in a given year, cars, properties, boats etc the IRS views that as 'dealing' inventory.
They may assign the activity 'dealer' status and tax it at a higher rate.
That's likely why the CPA on this thread, Sean has listed 'S-Corp' as a popular option amongst flippers. In the event that the activity is determined to be dealing inventory by the IRS, the total taxes on the profit are high... I think upwards of 40% depending on variables that a CPA can walk you through.
If you were doing passive investing and just starting out, in most cases an LLC likely isn't needed. Good insurance gets you what you need.
But for what you're talking about.... there is some risk with flips and an LLC isn't a bad idea.. more importantly... if you're flipping a lot there are tax implications with that. This is where an LLC that's taxed as an S-Corp makes sense.
But.... don't get carried away. If you only end up flipping one property in your first year just use a standard LLC. Make sure to get insurance too.
For the registered agent don't use yourself. Pay a service. They're cheap.
Hope this helps. Good luck!