Acceptable. Make it an acceptable offer though, I go for a little less than the average price of similar items. Don't insult their intelligence by completely lowballing them, they probably know what it's worth.
I use it pretty often when dealing with semi-rare stuff like older cameras, they (especially if they are not doing it professionally) don't have to deal with waiting around for a payment and paying ebay fees and I get what I want without having to spend a small fortune.
However, remember to use caution since it won't be through ebay...