1. You don't that that he hasn't filed. You know that you have not found the filing. (Those are two very different things and the difference is very important). There are a number of questions: Are you searching the correct court? Are you using the correct spelling? The correct name? I don't like absolute statements from novices because there are too many potential points of error.
2. If he filed bankruptcy, yes, you are vulnerable to a big hit.
3. When he files bankruptcy, the bankruptcy court will notify all the creditors listed in the matrix. If he fails to list a creditor, they won't be notified unless they check. Ultimately, in many cases, a creditor that isn't notified will not make a deal out of it because there are no assets to get. That being said, if he failed to notify you or list the asset and he filed bankruptcy, you both have a problem if the sale goes through.
Be sure you are right.
DC
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Edit to add:
You really should visit with an attorney. You mentioned payments. We don't know the basis of this sale, but it seems sufficiently complicated for the internet. You need to make sure you have a right to seize his property and sell it. Being wrong could get very expensive if he gets an attorney.