Hilchos Shemittah v’Yoveil 4:7

Regarding onions of shemittah that enter the eighth year, if there are comparable onions that grew in the eighth year, they are permitted; if not, they are prohibited. The same applies to other produce. They may not be purchased in the eighth year until eighth-year produce reaches the same size. When the eighth-year produce that grows quickly resembles the shemittah produce, the rest become permitted. One may buy eighth-year vegetables right away.

Hilchos Shemittah v’Yoveil 4:8

One is permitted to buy wild onions in the eighth year once the new produce outnumbers the old.