I agree with Dave, and would add one observation. Not all sellers price their items close to market value. It's not uncommon to see overpriced items, and in this case an offer of 75% might be very close to where these items should be realistically sold. However, there is no doubt that sellers of such items would consider 75% offers to be lowball. So, this becomes a matter of interpretation and point of reference. The educated buyer might believe the 75% offer is extremely reasonable, while the uneducated seller might believe the 75% offer is insultingly low.
In a perfect market, where items are priced accurately according to proper market value, I would consider any offer of less than 85% (15% discount) to be lowball.