AB and DT, thanks for the comments. I suspected that there may be a difference of opinions on this one.
However, I still think that taking a gun as an occasional fee is exempted from the definition of "engaged in the business" so as to require an FFL. The BATFE's own (CFR) definition even points out that "such term shall not include a person who makes occasional sales, exchanges, or purchases of firearms for the enhancement of a personal collection[.]" The fact that he takes the firearm as a fee for work performed in his business, as opposed to a flat trade for a lawn mower, is of no moment. I also think DT's got a good point that advertising "we take guns" in the local paper would probably attract unwanted scrutiny, even if it is later determined not to be engaged in the business.
I'm a lawyer, but I'm not your lawyer. If you need some honest-to-goodness legal advice, go buy some.