there is no right answer to your question. could be 1 really bad mile, or 100,000 really gentle ones. if it looks good and you trust the seller, go for it.
think about it...there are thousands of aluminum frames out there, many different alloys, lots of different designs. the answer is "it just depends..."
Miles don't really mean anything. With no crashes on nice roads just about any frame will have an almost literally infinite life. With a crash or mega pot hole you could damage on in a matter of feet nevermind miles.
3000 of ordinary use is nothing I'd balk at. Assuming no crashes and 'ordinary' use I wouldn't worry about the frame at all and needing a new chain is generally the only thing to look out for.
I put more than 3000 miles on my full aluminum frame in less than 2.5 months this year and am up to around 5000 so far on it with no issues.
As others have said...there is no real answer. A lot will have to do how it's ridden, who it's ridden by, how much they weigh, what type of power they are putting out, what the road are like where they ride, who built the frame, any defects in the frame, etc.
If you like the bike and there is no visible damage, it's likely going to be a fine bike for many thousands of miles.
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