If he still needs iron and horses, this is not in your favor. Strategic resources the AI consider useful are worth 45 gold per unit for 30 turns, while an even trade for a luxury resource is usually 240 gold for 30 turns. So this amounts to you paying 465 gold worth for marble... he doesn't like you, I'd say.
On the other hand, even when the AI considers the resources obsolete, they will try to yank away whatever you have that they consider "worthless" including open borders if you are on poor relations, just so that you cannot use it as a bargaining chip with someone else.
Any such calculation will always omit to factor in the +120 happiness to me (which will send me into a Golden Age), the 9 or so cities that will enter a "We Love The King Day" for 30 turns, the -120 happiness to the AI (as this was their only Marble), and the fact that with over 50 iron and 50 horses, taking 5 away from me is irrelevant - which is why the AI will always suck as it only makes decisions based on numbers and not context.
Oh, and next turn I got a DoF from them, so unless I'm about to be backstabbed for 15 gold ...
Each luxury good is worth 120 happiness?? What crazy mod are you playing??? They're worth 4 in my unmodded game!
All of the native American civs will backstab you eventually if you are close by and weaker, even if you have done nothing to provoke them. As a matter of fact, in my experience the only civs that won't do this are the Siamese, Americans, Egyptians and (strangely enough) the Mongols. They will just suffocate you by spamming cities instead.
This is assuming you didn't tick "random personalities," of course.
I don't know, I've started playing again and George Washington has really been getting up in my business for existing next to him. It's like he sees me as the Ohio River Valley and it's the French and Indian War all over again.
Anyways, I hate the AI's trade sense sometimes. They're more interested in sticking it to you than getting the goods they need. A simple luxury resource from someone who's neutral towards you is apparently worth at least twice that.
Of course, trading for strategic resourcs is patently stupid, since as soon as the deals over, you no longer have the resource and BAM, there you are with a big, honking penalty.
So much for the value of a barrel of oil.
Only if that's how *YOU* happen to want to do the calculation - like I said earlier, the value of a trade will *ALWAYS* be dependant on context - that same deal could also get me 60 culture if I had the required policy and/or a significant science boost if that 4 happiness per turn happened to take me from negative to positive overall happiness and again I had the required policy. Both of those have value, that your "calculation" fails to take into account. Assigning a purely numerical value to all items in a trade based on a fixed nominal "cost", without taking into account the current context, is why the AI will always suck when it comes to trade.