A question about the English translation, please: Does this just refer to the physical location of the wine? I thought if the wine was up for sale, it would rather be "on the market". Is the close translation making this weird or is it just me thinking too literally?
The Roman Forum was a plaza or square which functioned as the center of Roman civic life and contained many market stalls, so marketplace is a common translation if you don't just keep it as "Forum" in English.
If I recall properly, in Mediaeval Latin, the preposition can be used with the accusative. So either should be correct.
No. in (+abl): in, on. in (+acc): into, onto. In both Classical and Mediaeval Latin, when in takes the accusative, it's definition changes as well