I have three. Only one, a sporterized old '91, is due for the lamp treatment.
Ammo is readily available for the 6.5s. Norma, Prvi, and Hornady all make it. Well, as readily available as any ammo is these days. The Prvi stuff has bullet diameter that is too small for most rivles (Carcanos vary), but the brass can reloaded. Prvi used .264 bullets, Hornady - and I think Norma - used .268, which usually works better.
The MG ammo in 7.35 usually came on 15 rd strips, to be fed in Hotchkiss style.
Prvi supposedly makes fresh 7.35mm brass, but I haven't been able to locate any. Ammo can be made from 6.5s, but you'll need .299ish bullets, though Hornady makes (or at least has recently made) those.
Value? I don't know. Somewhere around 150 dollars still often buys a Carcano in many cases. Some of the old long '91s were shortened over the years, and those tend to be less accurate than other variants. An intact military rifle though, particularly a less common variant, could fetch 350ish, maybe more. The problem is finding a buyer willing to pay that, since Carcano's don't get a lot of collector's attention.
They're decent rifles. The basic design was in service for 50 odd years - 1891 to 1945, and saw use after that with Italian paramilitary/police forces.
I've seen a handful of sporters on auction sites that had the magazines altered so that they no longer required the clip. Not sure who did that or when, maybe some gunsmiths found a way to do it back in the 60s.
Copes, or Coles, or somebody, had the FNM made 6.5mm ammo cheap. Around what .223 FMJ is costing these days.
With a supply of ammo and a couple of the clips, would make a decent truck or utility rifle.
