Almost impossible to answer because the designs vary so much. After all, the model is usually the key factor in buying a used SMB. Sprinter vs E-series, EB vs RB, walk through vs a 50 type model. During my build SMB told me more E-series design builds change from the walk through models to the 50 model after people see both in person. Seems somebody around here made the claim that the 50 is the best model but it must be a green van
Just as a guess I'd say for an E-series the best re-seller would be:
RB-50 SMB 4x4, with a V-10 that has low mileage in a neutral type color (white, or light brown colors) outfitted in a standard package with pre-wire solar and an electric top would sell quickly provided it's priced low enough.
The standard package usually has a PH bed, sink, water tank, Propane stove/heat, small refrigerator, PP, 220AH battery, microwave and inverter.
If I were doing a walk through, the same package as above in an EB is probably the more desired setup.
Of course everybody has their own wants and that's why each is unique in its own way.