At long last avatars can be made to sit down in Sansar… see this blog post for details. Currently just cross-legged on the ground. Sitting can be initiated using “/sit” typed into local chat tool in desktop mode. A very nice lead in and return to standing animation is used that looks very natural.
The full list of “/emote” commands are given in https://help.sansar.com/hc/en-us/articles/360000286366-Avatar-emotes. You can exit sitting (and other animations that play indefinitely) by using a motion control to move your avatar.
Its not currently possible still to sit on chairs, sofas, vehicles, etc. There is a good discussion in Inara Pey’s blog post on 31-Mar-2018 relating to the technicalities behind this.