Can a co-owner sell their share to a stranger without telling the other co-owners?

Legally, yes — Israeli law does not give co-owners a statutory right of first refusal over another co-owner's share. A co-owner can sell their undivided share to any willing buyer without notifying the others in advance, unless the parties have signed a co-ownership agreement that creates a contractual right of first refusal. If you want protection against an unwanted third party joining the title, this must be built into a written agreement and ideally registered as a note on the Land Registry record.