You can't "engage in the business" of buying and selling guns. BATF deliberately has not defined that term because there are times when some obvious hanky-panky is involved, even if it is only one gun. Most of the time they don't worry too much as long as proper records are kept. But remember they check the sellers' books and if one collectors license holder comes up repeatedly buying a bunch of guns of the same kind, they are definitely going to check.
Also remember that any sales to non-licensees have to comply with state and local laws. In addition, some states do not recognize the collectors license at all, either for some guns or for all guns. In those circumstances, selling to another C&R licensee in the same state is the same as a transfer between individuals; you may have to go through a dealer or obtain police permission. To top things off, some states consider receiving certain guns in interstate commerce illegal for other than state-licensed dealers.
If I were a collectors license holder, I would not selling any guns to my "buds" or anyone else. No matter what excuse I would give ("Of course I ordered 2000 pistols, but I kept the best one and sold the rest to improve my collection...") I suspect I would not keep my license long and maybe spend a neat vacation at Club Fed if any of the buyers were wrongo or if one of those 1999 guns I sold was used in a crime.
The antis are very much determined to eliminate the collectors license; it would take only one incident to get it gone.
Jim