How much for a Used Red Sea Max 650S

[LayT27]

Someone I know is getting out of the hobby and I bought some coral from them at way below value. They told me they are planning to sell there tank too. Question is, what’s a fair offer for a 9year old Red Sea 650S (170gal). The tank looked brand new and comes with three Gen 3 lights. They are doing their research to see what to ask for it and I’m doing some too. Don’t want to low ball but I do want a sweet deal lol