He anticipates the seller's going to lose £50 (4)
I believe the answer is:
seer
'he anticipates the seller's going to lose ps50' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'he anticipates the seller's' is the wordplay.
'he' becomes 'See'.
'anticipates the seller' becomes 'r' (I can't explain this - if you can you should give a lot more credence to this answer).
'see'+'r'='SEER'
Can you help me to learn more?
(Other definitions for seer that I've seen before include "One divines the future" , "Person with unusual powers of foresight" , "One who knows the future" , "One fortelling future" , "I envisage" .)