I have to agree with the notecard thing. The chorus of ping ping ping ping!!!! <Too Many IMs, messages capped> happens to me anytime I'm off
SL for more than 12 hours or so.
The one aside, if you see "so and so has accepted your inventory offer", it means that they have probably logged on and will take a peek at it when they can. It does not mean one ought immediately IM them, then repeat the IMs with increasing hostility/sulkiness/both until the recipient has to stop what she logged on to do with the less than one hour she has before going to work, just to pay attention to your complaints that she isn't dropping all her current projects to take a commission from you to build something that isn't even vaguely like anything else she makes. By the time she mutes you, she will be far too annoyed to get any work done, and so cross that she snaps at her puppy, who turns on the sad eyes, and then she has to deal with the ensuing guilt all day.
Um. That was something of a random example.