Check the stopper separately
Ask whether the stopper shown is included and whether it is described as original to the decanter. Record the height with and without it, along with the maximum width. A similar-looking stopper may fit poorly. Do not force it into the neck to establish compatibility.
Document material and attribution
Request views of the base, decoration, interior and any marks. The former address mentioned Bohemian crystal and Czechoslovakia; those descriptions require supporting evidence for an individual object. Brilliance or apparent weight in a video cannot identify a maker or establish the composition of the glass.
Decide between display and service
Ask about chips, scratches, internal marks and previous cleaning. Examine the rim and stopper contact areas closely. For restaurant use, establish suitability for the intended contents and conditions instead of assuming an old decanter is ready for service. Where this remains uncertain, use it as a decorative object and describe that purpose accurately.
For the next part of your selection, read Silver plate marks and condition: a buying guide.