Dishonest buyer cost me money, is legal action likely to be successful?

the_lunatic_is_in_my_head

Assuming a private sale* relist for best price you can, if it sells for more knock the extra off the £42 and claim for the difference.

If it sells for less then add the difference to the £42 and claim for that. 

The threat might be enough, there are companies online that send a LBA for a small fee. 

Photo of it on the floor with no cables attached hardly paints the buyer in a good light  

Question of reward vs headache really. 

*If you are a trader (or selling equipment belonging to your trade) and they were a consumer they would have the right of return anyway which cancels the first £21, return would depend on info supplied. 

forgotmyname

If it's the priciple you could post the monitor to ebay and ask they where the damage is and why it does not work.  A nice note attached
stating please plug it in when testing because the lying buyer did not...

Watched a GeoffBuysCars video earlier and he was with a guy whose name I forgot.  Anyway the guy was making a claim from the council
for damaged wheel(s) due to potholes.  They asked for proof and he posted the actual wheels to them..