I feel more comfortable to invest on tlds that actually sell to endusers. For me buy low and sell high, mean buy undervalued and sell at a profit price.
At the moment nGtld are too much a speculative investment and I think this after watching what new start ups are buying.
There are many lists on dngeek.com
Yes they are all sold out. They sold out about a year ago. The ones without vowels or "V" in them are worth the most. You can check the charts at http://shortnames.com/llll/charts to see their current values.