I see they use “sold by the pound”, but can I say “sold in pound” instead? Also, they use “measured in pound”; can I say “measured by the pound”?
When using the version with in, the units should be pluralized:
Plums are sold by the pound
Plums are sold in pounds
Bulk nails are measured in pounds
Bulk nails are measured by the pound
Note: While "sold in pounds" is not wrong or ungrammatical, it's not nearly as common as "sold by the pound," which seems more idiomatic.
On the other hand, "measured in pounds" seems more common than "measured by the pound".