LP85's are $280 a piece brand new, a thermo manifold is $250 and DGX Premium bands are $100 but I imagine that another brand would be double so $200, or around $1,005 with 7% sales tax. Granted, some stuff has come down in price and DGX tends to be low price, I guess I would find $1200 a reasonable price but I still think $1500 was a bad deal they gave you.
As I mentioned, doubles sell for $400-$500 in hydro all day long. You might be able to sell out of hydro for the $400 price, it's a range after all, but it does line up well with the 50% value another poster posited.
If the OMS manifold is the face seal type, that's no good.
I personally don't have a big problem buying them out of hydro at all as long as there's a refund plan in place incase they fail. But that's rare, probably inside they are fine, and I don't think you will have anyone asking for a huge discount for that, but some will. Now, I did just pass on a tank that I knew was used in salt water, and the seller refused to take the boot off to check paint condition. But, I bought another identical tank under the same conditions last year, because the price was $20 and the seller was down the street. This time the price was $50 and the seller is an hour away, so I'm not going until he takes the time to take the boot off.