It's pretty common to approached out-of-the-blue on LinkedIn. I've been approached many times by people I've never met and have no connection to whatsoever. Always be courteous and polite in any professional setting!
"what's your current earnings in your country"?
There are several approaches to answer questions like this with widely differing opinions. I have my own personal preferences. In this case, I'd treat it just like you would in an everyday social setting. If an acquaintance (or anybody I don't really know very well) asked me what current earnings were (salary, hourly rate, annual income, whatever), I'd simply reply with something like:
"Well, the typical salary range of an X [occupation] with Y years of experience in the Z [geographical] area is $A-$B."
No, I don't really think it's anybody's business what I earn, and just because you're dangling a job in my face (out-of-the-blue, remember) doesn't mean you get to know, either.
To answer actually this question (if you even answer it at all), I'd at least wait for the other person to really get serious about a job offer, like proposing an interview (even a telephone interview). That shows they want you for your skills, and not based a range of income you fall into.
I am not really interested in negotiating, and based on my salary, I would get a little more. That's the feeling. Should I decline such job offers with such thrift?
I take this to mean that you think the job likely offers more money. Given the other details you have, decide if it's right for you. Only you and the recruiter can decide whether this job is the right one for you. If you aren't interested in negotiating, though, I don't think you're really interested in exploring other opportunities. If that's the case, don't waste your or the recruiter's time and politely recuse yourself.
"...Thanks for engaging with me about this opportunity, but at this time, I'm not interested in pursuing other employment opportunities..."