Hilchos Bikkurim 10:9

If a Jew buys the shearings of a non-Jew’s sheep after the non-Jewish owner has shorn them, he need not give first shearings from it. If he buys the non-Jew’s sheep for their wool, he must give first shearings even if the wool grew while the sheep were in the non-Jew’s ossession and the sheep were returned to the non-Jew after shearing them. Since the one doing the shearing is Jewish and he owns the shearings, he is obligated. This is because the obligation is only activated when the animals are sheared.

Hilchos Bikkurim 10:10

Let’s say that one Jew buys the shearings of another Jew. If the seller kept some sheep to shear, he (the seller) must take from those sheep enough first shearings for the whole flock. This is so even if the seller has not yet begun to shear his sheep. This is because we assume that one doesn’t sell the gifts that must be given to kohanim. If the seller didn’t retain any sheep, then the buyer must give the first shearings.