I'm sure it varies from collector to collector, from item to item, and from offer to offer. I have sold items to a fellow collector friend before now for less than I might have made elsewhere, but neither did I give them away. 'Mate's rates' are not unusual, I'm sure, but normally if I have something of value to sell, I try to achieve a fair price for it that doesn't con either party. I want to be fair to myself, but not being a greedy scalping asshole, I certainly don't want to rip anyone else off either. And since discovering this Forum, I would ideally try to sell whatever I had for sale on here first so other collectors stand a chance of buying it.
Money is important (all the more so if you don't have very much of it, as you usually appreciate its value more), but it isn't everything either. Selling on this Forum at least gives you a measure of control over where your item ends up, as you are under no obligation to sell to any particular interested party. I would, for example, prefer not to sell anything to anyone I felt might be a scalper trying to turn a quick buck, or to someone I felt didn't deserve it or who wouldn't truly appreciate it. I would always prefer to sell my item to a good home where it will be appreciated.
Others may disagree of course, but that's my opinion.