There is a thing i cant rly understand.
Poor technical choices.
The player who launch the dungeon / FOTM / instance is hosting everybody else in its personnal instance. If he gets kicked (or, for example, leave the group by himself), that personnal instance is ended and everybody else kicked out.
I have the feeling that this would be a difficult thing to correct, technically speaking, for ArenaNet (otherwise it would have already been corrected months ago).
And yes, it’s really a bad poor choice and should never have been coded like that. There are numerous players that have been annoyed by this problem.
Dont sell paths alone, get a friend to stick around until you get full party. The best optimal way is to solo the dungeon, and before you start selling, fill the party all the way up to 4/5 with guildies or friends and then start LFG. This will prevent your party from being merged and ending your instance. As soon as the buyer joins, kick 1 guildie and repeat until its full.
I have only once used the merged party function to kitten someone’s instance. And that’s when he stated he would pay 1,5g for help at tar Arah p1 and then he and his guildie kicked everyone that helped without paying the cash. I memorized their names and once they listed their LFG with intention of selling the path, I ended their instance.
Pay what it’s due.