Need a guidance where to get or find or do, the how to mark an Item for SOLD,
Thanks, MB-Themes.

Some themes has it prebuilt.
Regarding plugin, I think backoffice manager has status functionality.

Thanks,
I have seen that plugin require some PHP/HTML/CSS Requirement, to Install, is that correct or I misunderstood.?.

I just saw this line of code at the Demosite,

Do not forget to add code to get Republish Link in user backoffice - on user items or in dashboard. This cannot be placed using hook and needs to be done manually placing function to correct file.
<?php if(function_exists('republish_link')) { republish_link(osc_item_id()); } ?>

this code didn't add where to add this code , i mean which file to add this code, some other functionality like feedback it's mentioning where to add (file....php) this code

is there a full documentation a guide for this plugin.

Yes pligin require basic knowledge.