Where are there used copies for ~$300 or less? The used copies I see on POTN are at least $350, with most leaning towards $400.
I always buy used stuff on CL. You can get good deals on POTN or eBay, but the really great deals are usually local. I bought a used Sigma 10-20mm for Nikon for $250 a few weeks ago from a woman who lived 5 minutes from me.
The issues with buying on CL are threefold:
1) Availability varies greatly by area. You need to live in a decent sized metro area to find anything (1.5m+ population).
2) You need to check often. I make it a habit to give the listings a quick scan at least a couple times per day.
3) You need to reply FAST, especially for the great deals. The great deals are usually already scooped up within an hour after the listing was posted (I replied for that Sigma 10-20mm about 15 minutes after it was posted and picked it up an hour later).
For the price mistakes (e.g. some old lady selling her husband's old lenses for a fraction of their actual value), you need to reply AND pick up the items within an hour. The problem is that sellers usually get tipped off to their pricing errors from 'good Samaritans' (other buyers who are bitter about not being fast enough to get the deal) or from the large amount of e-mail traffic their listing generates. I've driven to the other side of town late at night to score some price mistakes.