Hard to put a percentage on it. If I'm selling a $3 cart and you offer $1, I wouldn't really consider that a lowball, it is a common cart anyway.
But if it is a $100 item and you offer $33, I think that's a definite lowball.
I go by "feel" rather than measuring value. If you sent me an offer and it feels like a lowball and you have little to no feedback here, you're probably getting the ignore treatment from me. If you are an established collector that is new to a platform (maybe just started N64) then I'd respond appropriately.
But believe me, when you've had plenty of window shoppers or lowballers or people who want pictures from 10 different angles only to say "not interested", you don't have the patience to respond to every single one.
It is also how it is presented. If you say "How about $20?" for something I think is $100, probably no response. If you say "Hey I don't know N64 too well, would $20 be acceptable?", then I'll give you some time of day and say why I think it is worth $100, maybe condition or rarity or both.
The longer you are in the hobby the more keen your eye becomes for time wasters and lowballers.
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