Note: The logs attached in this post are probably not all that useful as they are from after I fixed the issue and re-launched the game.
(What I did to fix the issue is below)
Backstory:
After extracting from a raid on Customs in which I killed a PMC with an M1A with 50 round mags full of M993, I unloaded the mags into stacks of 50, which I believe isn't actually possible, so I split them up and put them into their actual stack size of 40. I then went to sell those stacks on the flea market, where I noticed that the offer never actually listed properly. Ragman then tried to give me back the stacks of ammo, but upon trying to receive the items my game got stuck so I hard quit and restarted. I then encountered the error in the first attached image when trying to launch the game.
This is likely the same thing as this post: Error converting value {null} 'System.Int64'. Path 'ragfair.salesSum', line 1, position 402284
In that post it is recommended to delete all of your flea market offers, which I did, but that did not fix the issue. I do not know if it is actually necessary to delete the offers if you are trying to maintain your profile to as close to what it was before this error (It might still be a good idea - choose at your own discretion).
Fix:
In order to fix this all I did was open my profile JSON at <Path-to-SPT-Install>\SPT-AKI\user\profiles\<your-profile>, Ctrl-F search for the phrase
"StackObjectsCount": null
and change the null value to the proper size stack for that item (see attached images for the differences). Do this for all matches on the phrase. You will need to know the item's _tpl ID if you want to figure out what item is actually giving you problems, so just copy the "tpl_" string and google search for it and you should be able to find what the item is and what the proper stack size for that type of item is. For me it was a stack of roubles for some reason (maybe when it tried to take a fee for selling the bugged ammo on the flea) and the M993 itself in the Ragman dialogues for returning it to me.
Hopefully this helps someone, thanks for reading! 
