Is it not rude to offer an different price as long as it is somewhat reasonable however you as a seller can put firm price so its your own fault if you dont add it!
I believe you did mean to write, "It is not rude....correct?
That's the whole point. Lower offers are anticipated. Lowball offers are something else entirely. Then you open yourself up to, what you think is reasonable versus...
I say anything 15-25% of an asking price is utterly ridiculous. I wouldn't even make an offer like that to someone. I'd just move on to something else if I really felt that way.
The terms "lowball" and "reasonable" are all subjective terms. People that post about "low ballers" keep complaining that their listed prices (asking prices) are reasonable, yet they neglect to indicate the metrics of the listed/asking prices.
An item is only worth what someone is willing to pay for it. The app is free, so there is no loss for a seller to wait however long it takes to sell, which is fine. However, don't complain about the undesirable offers, when the app is free.
Perhaps, if/when OU decide to charge listing fees, the asking prices will mirror what people are willing to spend, since sellers will have to face reality with the costs involved.
On another subject, I like the "big box store" ads on OfferUp, since it can used as a comparison tool for similar items posted by private sellers who over value their used items.