Value the van without the WC system inside, would need more details about the van, year & more details about the model specifications, easy to value that by similar on-line comparisons.
More details about the Grippa system, one man or two? delivery only, DI, full Ro?? age & condition? do Grippa still make the exact same system you have in your van or have they updated it to a newer version?
Advertise it early as you'll have tons of time wasters & tyre kickers, I've been there I know.