Unable to wind 861 Speedmaster

I recently bought another 861 Speedmaster.
The seller described the watch as service history unknown but fully functional.

It’s now arrived and hands can be set without issue but the crown won’t wind. Specifically, it feels like its already fully wound - I.e. I can add a quarter turn to the crown but when pressure is release it reverses by the same amount.
At the same time, the watch is dead. Hands don’t move and the hairspring isn’t moving.

However, if I turn the crown by that final quarter turn and keep pressure applied to prevent it from reversing, the watch fires into life and the seconds, minutes (and presumably hour) hands are ticking away.

Any thoughts as to what could be causing this?
Could it be a simple fix?
Should a full service resolve the issue?
Is it indicative of an issue that might require expensive parts at service?

thanks

It's just impossible to answer without having the watch open on a bench... A full service is probably in order.

Thanks for the replies both.

I assumed it would likely be a case of “no one can give any real diagnosis via the internet” but I wondered whether the specifics about it starting to run once I manually apply rotational pressure to the crown would point to a specific issue.

I couldn't guess the specific problem, but I have bought many watches with movements that were jammed like that, and as far as I can recall, they all started running after being cleaned. Of course, the fact that it's not running means you can't test the other features of the watch, so there could always be additional issues that appear.

Thanks Al, I wasn’t aware of that and it’s good to know.

If the watch was sold as 'fully functional' and it doesn't work you could always ask for a refund. Or if you like the watch just do a full service on it.

That’s what I’m currently deciding on.
The seller insists it was working and their pictures seem to back that up. However, It certainly wasn’t working when it arrived as I always open the package, give it a quick, 30 sec cosmetic once over, then wind it/set the time etc, before a more thorough cosmetic inspection.
In this instance, I realised within about a minute of opening the package that there was an issue.

I generally budget for a service anyway but in this case I was expecting/hoping to get it done more in 6-12 months. Equally, I don’t want to decide to keep it and find the movements a melon and requires a really expensive service.

I’ll simply have to decide how much risk I’m willing to take in this case and make a call one way or another.