For a reasonably common current model machine in decent clean condition that can still be got new I agree half new price is reasonable expectation. Possibly a little more.
Bidding wars are for rarities or obsolete machine of high reputation. Generally at well less than half new price as such things tend to have been well beyond a private persons pocket when new.
The big imponderables are transport and loading.
If the machine is only a few miles up the road standing in a clear garage with easy access for the trailer or truck with a couple of fit healthy guys on hand to help load I'd pay a bit over the odds. Saturday morning job!
If I have to drive 400 miles (Manchester maybe?) and dig it out of a cluttered shed with only my mate to help me its going to have to be seriously rare and highly desirable if I'm going to pay a good price. Realistically thats £200 + in fuel, food for me and mate, a very long day and either a decent wodge to my mate or serious favours owed.
Knock £500 off.
I'd give Facebook marketplace and some of the local buy and sell groups a try first before E-Bay. Facebook tends to be much more local and there is less competition. The "Hmm not to far" effect is valid for turning "Maybe I'd like one" int "Stuff it I'll get it". I've occasionally gotten lucky but E-Bay is more relatable.
Clive