Just service them yourself - fit new inflators if time expired, check the cylinder weights and give a 24 hour inflation test using a compressor rather than blowing them up by mouth as that introduces moisture into the bladder. I do ours every year as part of the annual service/safety inspection. Mark the weight on the cylinder and fill in the service tab inside the jacket . You should get 10 years if you look after the jacket. There are plenty of 'how to' clips on YouTube.
I am not sure about auto inflation for a hovercraft - certainly in a boat but, if you get trapped under a craft it could cause issues if it auto inflates. I've gone for manual on the craft and auto elsewhere.
A crotch/castration strap is absolutely essential. It stops the jacket coming over your head and floating away.
I buy Crewsaver equipment. They are a big commercial brand and their equipment is hard wearing, intuitive and comfortable to wear.
Ebay life jackets

.... would you buy an ebay parachute ?
Don't buy second hand stuff - you have no idea if it has been abused and up to your neck in freezing water is no time to find out :-)
Just my thoughts. :-)