and how can I distinguish NPC --- Lost Soul --- Player that don't play and didn't drop his character. does their names will change, they will behave differently? for example I visit some abandoned station, I see character in there. what should I look for to say "Yes, this is a Lost Soul"
As to what you can do with them, they can be used as regular NPCs, hold gear, be part of a squad, level up to level 10 (if they're not there or higher already), and so on.
What most buyers want with LSs is their role-play value. I currently have a number of LSs, some of whom had fascinating IC history and events attributed to them. Having control of their presence means I can RP their knowledge and experience to support my own IC experiences. Feel free to check out my Holocron, where I mention these characters and their current roles in my own small army of supporters and troopers.
I should also point out, as far as I know, a corpse will decay in 30 days. So yes, as Jic mentioned, you can claim the corpse, but it won't stay around forever.