I recently volunteered to play a major role in a project that the organization could have hired out instead. I cheerfully and willingly offered to contribute significant hours gratis, but the leaders were hesitant to accept.
"I don't want to take advantage of you," one said.
I don't want them to take advantage of me either! (Although it seems hard to take advantage of someone if they offer of their own free will...)
It seems that the key distinction in this scenario was that we were taking advantage of the opportunity that allowed me to be able to be involved in this way. I had both the ability and the availability at the same moment as their need, and it was because of this juncture that I raised my hand.
When someone makes you an offer, assume that they genuinely want to make it. The person knows their circumstances and commitments and can best judge how their proposition fits within that context.
Just as with a gift or a compliment, a present of one's time should be greeted with a polite "thank you" instead of "oh, you shouldn't." Grace in receiving is a gift, too.
-- beth triplett