A wool-stapler is a dealer in wool. The wool-stapler buys wool from the producer, sorts and grades it, and sells it on to manufacturers.
The expression wool-stapler fell out of use during the 20th century, see the external link below. Staple in this particular context means a market.
Prior to the 17th century Staple was also a particular type of market, "a place appointed by royal authority, in which a body of merchants had exclusive right of purchase of certain goods destined for export".
The now best known English staple was at Calais but in medieval times there were, at various times, many others throughout the kingdoms of England and Ireland and the facing coast of the Low Countries all involved, though not exclusively, with the English wool trade.
- Oxford English Dictionary entries for 'wool-stapler' and 'staple'.