For example in the market, I was buying pork ribs at the counter and the attendant who has the only access to it was making me choose which pieces I want. I remembered the funny video "Is mir egal" so I just said "egal", but looking at some usages of that word in youtube, it seems more closely translated to "I don't care". Does it sound rude when used in this context?
What about this?
Irgendetwas ist okay
Thank you!
The German word "egal" was brought into German at about the year 1650 from the French language (égal = equal, the same), but it originally was a Latin word (æquālis = equal, of the same age, of the same kind, uniform, even). Also the English word "equal" derived from the same root, but also "equator", "adequate" and "equilibrium" (in German: Äquator, adäquat, Equilibrium)
So, the meaning of the German phrase »Das ist mir egal« is:
German: Das ist mir egal.
English:
This is the same to me.
This appears equal to me.
To me it looks the same.
etc.
But of course the meaning "I don't care " also resonates. But this is not meant rudely, and a native German speaker will under normal circumstances not understand it as rude when you say "Das ist mir egal".
If you fear to be understood rudely, you can replace "egal" by "nicht so wichtig" (= "not so important") and you can even soften the meaning by adding some extra stuff:
Ach wissen Sie, das ist mir eigentlich nicht so wichtig.
Oh you know, that's not so important to me.
In all these cases it's important to add the word "mir". This is a personal pronoun, first person singular, in dative case. You can add it to almost any German statement (as a "free dative", you can search for that term), and it means roughly "to me". With this word you can turn any statement that otherwise sounds as if you would claim a universal and verified fact into a personal statement about how it impacts you. When you use this word, you don't say that it is an obvious fact that the two pieces are equal (which might sound harsh), but you say that it makes no difference for your personal needs.
Irgendetwas ist okay.
Don't say this. This sounds like a very weak direct translation from a foreign language. No German native speaker would say this. The sentence is grammatically correct, but it doesn't mean what you wanted to say. The word "irgendetwas" does not refer to only the two pieces that were offered to you. It in fact refers to any thing that exists. Even in the context you told us it still can mean the butchers apron, the car parking in front of the shop or the next streetlamp (although this makes no sense when you are obviously there to buy meat). The word "irgendetwas" really means "anything" in the sense of any imaginable thing. It does not mean "any of the offered items".
You can actually use the word "egal", without sounding rude, if you wrap it in a sentence. Generally speaking, most words would sound rude, if they are on their own. However, if you are looking for a nice way to phrase that you would be okay, with whatever that person chooses, I would recommend either something like "Das sieht alles gut aus, Sie können gerne einfach eins aussuchen." Or something like "Einfach irgendeins." would be more common. Your suggestion with "Irgendetwas ist okay" would be understandable, but not as smooth.
Neither rude nor super friendly but definitely suitable to survive the store:
Das überlasse ich Ihnen
(I leave that to you)
Das dürfen Sie bestimmen
(You can decide that)
Another option in the same vein as @Pollitzer's answer: "Was würden Sie empfehlen?" ("Which would you recommend?").
This has the advantage that you express appreciation of the attendant's judgement, which might get you the best parts (except in bad shops where they might give you the stuff that otherwise no one would want :-) )
In addition, you're the one who makes the final choice, which fits your role as customer.
