Simply, imo, if an ancom wants to try and take my 'property' from me, they are using force. So I feel perfectly justified in resisting and matching that force.
Here's the rub to anarchy. At the end of the day, if you can't protect your shit, it might not be your shit. But that's just as true today, is it not? If you can't stop me from stealing your car, it won't be your car for much longer if I so choose. It might get found, you might get it back, but while I have it, it's mine as far as I'm concerned as a carjacker, yes?
I'm not saying property doesn't exist, but there is something to be said about your power to possess something being the only real indicator of 'ownership'. If you can't keep it, you might not keep it.
But yeah, just because we work together to end the state doesn't mean it'll be all peace and love and shit. It means that we'll have to repsect each others' boundaries and deal with the moments when they clash. No different than today but that there won't be assholes running around making shit worse than it need be because 'they are the law'.
This also supposes that ancoms and an-markets are 'enemies'. I think they are simply different ways to interact with their communities. At the end of the day, they are both about resource allocation and the methods to facilitate such. If you have an enemy simply based on how they choose to trade and allocate their resources with others... well, that's kind of a petty thing to have an enemy over.
The fear between the coms and caps is the same on both sides: that one will use their system of resource allocation to force the other to abide by their own preferred system. It's assumed that one will try to FORCE the other.
If you believe, first, in voluntary, consentual interactions and exchanges, then you recognize this can't be done by either side and any attempt to do so would be rightly met with defensive force.