There are multiple ways of doing it. One would be using advanced filters:
Make a copy of the sold items list and rename the header exactly the same as the Stock (SKU) list:
Now, from the DATA tab, select advanced filter:
Then, select to filter in place and fill in the criteria range:
Press Ok and it will show only the sold items in the stock items list. Delete.
One other way could be to use a VLOOKUP to search the stock items on the sold items list. All items that were not sold will return an error... filter the errors out and delete all remaining (see attached excel sample file)
If you wish to use the VLOOKUP approach then place the VLOOKUP in a helper column (I used "G") and filter the errors out (see attachment).
The resulting list is the items that were sold... delete them or do whatever you wish at this point: