If he'll open one up and let you check out the headstamp, you should be able to identify it. Guat is headstamped IMG which stands for Industrias Militares de Guatemala. My guess is that word is in Spanish...or Portugese (not french). If it's 7.62x51...I'm not sure it's Guat, I've only seen 5.56 IMG BP's.
Chances are he isn't gonna be too keen on opening up a battle pack so you can see the headstamp. If he will open one for you,
this website should help you ID the headstamp:
http://cartridgecollectors.org/headstampcodes.htm
If you can't find it on that site, chances are someone on here will be able to ID it for you....the key is to find out what it's headstamped, and post a pic. Good luck! If you're able to ID, share with us what it is and what you paid...or what he's wanting.
I'm looking at a 200rd Port battle pack right now. Dark green vinyl with a white sticker that says:
200 ROUND 7.62x51mm , with a black dot then the Nato cross. Under that it says BF 80––27. I've got other Port marked FNM on the headstamp. Nowhere is the word "cartouchos or cartouches"
FWIW: Recently it seems most Berdan primed 7.62x51 Nato milsurp is bringing around .50 cents a round, with boxer primed going up to .65 per rd. Match type like PRVI and Black Hills will bring up to a buck a round or more. Problem is the market is totally upside down now. Half the battle is just finding ammo available to purchase no matter how it's packaged or primed. When the demand exceeds the supply...like it obviously is now....evidently the sky is the limit. Check the EE on this site to see what 7.62x51 is bringing. What one guy is willing to pay doesn't really mean thats what it's worth. If you can buy this stuff you're referring to around .40 a round or cheaper, I'd jump all over it. At least it's not Indian crap. South African has been bringing more than than 50 cents a round for quite a while now....thats berdan primed 30 yr old ammo (still decent ammo I might add, just expensive as those 140rd BP's originally sold for $29.95). I'm not saying it's SA, (it's not, that is packaged 140rds to a BP and nowhere do they use the word "cartouches or cartouchos) I'm just using it for an example.
A picture might be worth a thousand words. If he won't open one up, offer him $70 per PB and go from there.