Hi Tony,
the ARAS forum has a new for sale section now, “Petites Annonces” though there are not really many constructors there.
The astronomical_spectroscopy yahoo group probably has more self builders and I see there was a recent ad there for an L200 for example, you could check with Ken who owns the group if it would be acceptable
Also Stargazers Lounge perhaps where there is some interest in spectroscopy ?
Robin