Ah, one of the eternal questions; quite a few past threads on this.
As everyone is noting, this is pretty subjective, but I'm quite sure that 75% is too high for the lowball line; for example, pristine dealer demos, b-stock, or just sale items often come in with markdowns more than 25%, with warranty. I think it pretty reasonable to look for 60% of msrp for used electronics, and sometimes less for hard to ship speakers.
I've both bought and sold for 60% with no cries of lowballing, but this may be due the fact that I tend to ignore adds with things like "lowballers will be politely ignored," "no tirekickers," etc, because I prefer not to trade with people who initiate transactions by insulting potential buyers. If the seller has a firm limit, they should say so. If the seller doesn't want to list a firm price (thus leaving wiggle room in order to maximize their return), they should entertain offers.
Just my 2c. Takes all kinds.
John