I’m new to unicycling so I’m not sure if this unicycle is any good. Does it look like it’s worth buying?
Depending on how much it is, and depending on if you only want it to learn on.
It doesn’t look like the nicest thing to ride but if it’s cheap then it could be ok to try out unicycling on.
If you want something approved… then the Torker LX20 is a popular choice.
Is the torker LX 300 or 400mm?
Which part of it?
If you mean the seatpost, it doesn’t matter because unless you are really tall you will cut it anyway.
How do you cut the seatpost? My inseam is about 31in without shoes, is there a seatpost size that I wouldn’t have to cut?
It’s easy, just with a hacksaw.
Getting a longer than necessary seatpost is always a good idea because then you can cut it down to the perfect length.
-Miles
That appears to be an old DM Ringmaster in the picture. They are well made if old school unis.
If you want to learn trials or something extreme then forget it.
If you just want to learn to ride it would be fine if the price is low enough…
They are not as crumby and uncomfortable as the old pashley unis 
Does this unicycle look any good? I can’t tell.
If the price is right: It looks like a cheap one, but there aren’t many details to go on in the picture.
Torker Lx reply
if you have a local bike shop near you you can also ask them to help you cut the seatpost to your hieght but you do not want to cut too much you just need to cut and make sure that your leg can reach it and will let your legs pedal straight
oh and make sure the legs are straight when unicycling