It could be grand larceny, a felony, if the police/DA can prove you intended to steal the money. If you pay it back now I don't think you will have a problem assuming you can show you tried to return it before. The statute of limitations is 5 years by the way.
I am a former federal and State prosecutor and now handle criminal defense and personal injury/civil rights cases. Feel free to check out my web site and contact me at (212) 577-9797 or via email at Eric@RothsteinLawNY.com. I was named to the Super Lawyers list as one of the top attorneys in New York for 2012. No more than 5 percent of the lawyers in the state are selected by Super Lawyers. The above answer is for informational purposes only and not meant as legal advice.
My guess is that this would not be considered a criminal matter but rather a civil one. Whether purchaser can find a sympathetic prosecutor to go after you or not, would be one question. Why you would want to even go there is beyond me. I would see if you can work something out with them to avoid ANY kind of court. I assume that you cannot pay them back. But you should try to arrange to pay them back in installments. I do not see any way you would be able to defeat their claim.
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By not selling your mobile home to the prospective purchaser or returning the entire balance you may be charged with grand larceny which is a felony punishable by a state prison sentence. However, in matters like this most prosecutors will generally not prosecute deeming it a civil matter.
The one thing I don't understand and could cause a prosecutor to decide to prosecute is that you returned $500 and claim you could not return the rest because you could not get in contact with the purchasers. How did you return the $500?
I suggest that you contact an experienced criminal defense attorney in your area with whom you can discuss this matter, who can review your documentation and propertly advise you.