Most of the folks I know, who aren't FFLs, but who buy and sell more than a one or two firearms in a year, were FFLs, but had their licenses renewals declined, because they "weren't in the business".
Suppose, for the sake of argument, that this guy falls into that category. Would his having had his license pulled because he wasn't in the business be considered a defense against charges of engaging in the business without a license?
Probably not...