Selling at Less-than-Vendor. Why?

So I had a few Battlecraft Demi 2s up, and last week they were going for about 700 each, and I check my retainers today, and noticed they were still up and so I checked, and I knew they got undercut, so I was curious, and I check, and....

260
280
280
280
280
280
299
300
310

...... and I'm like "meh, whatever. What's the vendor price?"

......300.

I'm like "the heck is the point in selling junk for less-than-vendor?" I could understand charity of selling for break-even (which is basically giving it away for free), but to sell at less-than-vendor, you're actually eating a loss because of the fees involved with selling. I forget what the fee is, is it like 5% or some-such? So if you sell a 300g item, you're only going to be getting 285g. Or, you know, you could have just sold it for 300g at the vendor.

I'm not complaining or anything, but I've noticed this before and I just don't get the mentality behind this. If they are so easy and so low demand that people are willing to give them away, then why not just vendor them?

not paying attention. reading is hard.

They do it all the time.
A possibility may be that they don't know it's the vendor price, the default one, and think it's the advised prices, since that's what other MMOs do.
Another possibility is that they want to just get rid of it no matter (like most people say they do) so they just don't care.
Or they want to keep people from actually making any sort of gil out of it.
Or....they're just completely stupid.

I don't understand it either, as you can just have the retainer sell the item right there instead of listing it.

May be not paying attention or not understanding the interface.