I'm very new to the CS world and prefer vintage / antique items. It's around 5 mm, platinum with diamond melee, from the 20s (according to seller). It's around $300.
$300 is certainly a deal for the platinum setting with what appear to be 10 OMC diamonds, but the emerald itself looks to be of very poor quality. Ignoring the fact that treatment level is unknown (and therefore presumed high), the emerald either has a surface-reaching fracture running the entire stone from north to south, or it is actually cracked in half. If you like the setting, it would be worth it to buy it and replace the emerald, but don't buy the piece for the emerald, which is likely worthless and may well shatter from regular use and wear.
I'm not an expert, but I can see that crack as well. If the setting is stamped, it might be worth getting. There are great CS or labCS you could put in there.