I think it's going to go for more than $300... but just how high is anyone's guess.
How much would an early G212 sell for? I can't recall, because I haven't seen that many of them sell. Two D-40 six strings from the early '70s have recently sold on eBay for $580 and $1299. I would expect a 12-string to fetch more than a six string, and a D-40 12 string /G-212 to sell for more than the D25-12s that show up on eBay pretty regularly. Also, there might be some "uniqueness" to this guitar... it's a Guild dread 12-string from '72, which is a couple of years prior to the introduction of the G212 and G312. It doesn't say "special" on the soundhole label... but from my understanding, it wouldn't be a standard production model at this time (perhaps Hans could shed more light on that).
I don't think it looks to be in too bad of condition. Unless the lifting bridge indicates major structural issues (severe belly and/or split bridge plate inside), fixing the bridge shouldn't be a major expense. The seller says no cracks... but there is an odd linear element in the reflection along the neck in one photo that worries me... perhaps the "truss rod" crack that is seen in some Guild 12 strings? I would inquire about that... could just be an odd reflection. I would also certainly ask all the important questions about the neck and action before committing myself. The saddle looks nice and high, now... but the seller doesn't mention how high the strings are off the frets. I might even ask the seller to look inside with a mirror and report on the bridge plate... but that might be asking too much. Doesn't sound like he/she knows much about the guitar.
Good luck Dane!
Dave