First up, dont get me wrong, I am not trying to have a go at you, but I think chilling out on the negociations could help out
eg. They ask £250 you reply £2380, once I reply that I would forget about it as the opening price is not huge and if they want it they can do the leg work.
First 'mistake' in my eyes was to chase it up within 24hrs! Chuck them an email a few days / week later and ask if they are still interested , but not within a day!
They came back with £675, so starting to get worthwhile, then you dropped a grand - Mistake #2 !
They then went up to £950 , which you replied with £1200 + transfer fees.
You have to remember you dont know the circumstances, there may be someone else who has to clear or sign off on a purchase over a certain amount. They could reply explaining this, but buying a domain may not be their top priority.
Likewise invoicing their £950 despite them not coming back to you , possibly mistake #3
You have gone down nearly 1.5k in price in a couple of days, and invoiced on a lower amount etc. To me as a buyer that would make me think you are gagging for a sale so could accept less in the future.
Things can change depending on the buyer and circumstances, but at the end of the day the domain is yours and they can not have it so as long as your valuation is not wild usually a deal can be reached
Hope this helps!