If you could please send links to the auctions, it would be helpful. $1375 is very high...in fact, it's the highest sales price I've heard of for a normal Model 65. $370 is a low price, so you should feel good about it. That being said, the one that sold for the high price is within the known/likely USMC serial number range, and the one that you bought is not. It is generally accepted, though not exactly proven that the first 6000 Model 65's went to the USMC. Perhaps the buyer at $1375 knew that, and "had to have" a known USMC serial number range Model 65 for their collection. Of course, there had to be 2 bidders interested in it to drive up the price.
There were small changes made as production progressed, though there should be no significant difference between the two Model 65's that you mention.
One thing that can affect value is whether the rifle's receiver has been drilled and tapped, or otherwise modified later in its life. Also, some rifles have the wrong rear sights on them, or are missing their rear sight, and that can easily cost $150 to $200, and become somewhat of a quest to locate a replacement. Front sights were also a common after market modification. That's why the auction links to photos would be quite helpful to evaluate whether it was just auction fever, or if other factors may have contributed to the wide price variation.
Model 65's were not marked with USMC markings at the factory. USMC markings are unusual, and could have been applied by a USMC unit armorer, however some USMC markings we have encountered are almost certainly the work of a faker trying to enhance the value of the rifle, and any USMC marking on a Model 65 should be closely scrutinized, and is extremely uncommon. Again, these rifles were not USMC marked when they left the factory, with the exception of some later Model MC-58's, which were manufactured in 1958-59 for the USMC. MC-58's were Model 65's with the M1 Garand style safety in the trigger guard, and some had "USMC" incorporated into the overall model designation roll marked on the receiver.
Hope this helps...I sort of have a Model 65 affliction myself...