MI Homes has a program to help you sell you current home, but I am not aware of any builders who take another home as a trade-in. Existing homes are generally handled as contingencies in new home contracts. You disclose that you have a home to sell and the builder gives you some time to sell it. If you need to sell and are still in the market, please contact me.
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Regarding the new home you are looking for, I'm the sales rep for Byers Creek in Mooresville, a Standard Pacific Homes community that was recently opened in the area you are looking to buy. I'd be happy to show you what we have in your price range and further discuss solutions for your home you need to sell. Regarding our company, we are consistently among the top 15 builders in the nation for volume and consistently rated tops for customer service. In fact, we are the only builder to ever win the most prestigious customer satisfaction survey in the business (copyrighted name, so I can't say, but you've heard of them) for the Charlotte market 3 times (2006, 2009 & 2010). We have several floorplans that would fit into your price range, so I'll include the link to my community should you decide you want to check it out; thanks & good luck!