I have purchased several tanks on eBay and have got some excellent tanks at excellent prices. I probably bid on 15 tanks before I get one bought at a very good price, and I have also purchased 5 dry shippers on eBay . The dry shippers have cost between $1 and $310 and all are working fine. The one I purchased for $1 would not hold nitrogen so I had the seller send it directly to MVE to see if it could be fixed. It cost me $200 and they gave me a new warranty on it. So, I got a dry shipper that lists for $1050 here, for $230 , including the shipping here.
In your case, I would suggest that if you are going to be handling other people's semen, I would not mess around with a used tank. Bite the bullet and buy a new one. There are several good brands of tanks and most studs have them. Talk to the people at the studs and they can probably tell you which ones they like the best. They work in this business every day and they should be able to give you good advice.