If an average game lasts maybe an hour and you play it with an average of 3 players that's 3h of entertainment per play. 10-15 Euros per hour of entertainment seems very reasonable so most games actually tend to pay for themselves after just one play if oyu look at it that way.
I've had my copy of Dominion for over three years. I got it on sale for $30. I've played literally hundreds of games since I bought it.
Granted, I've added expansions over the years. But let's assume I paid full retail price for the game and four expansions: around $225. I've played at least 300 20-to-60 minute games. Conservatively, 100 hours of gameplay time. $2.25 an hour to entertain myself and 1 to 5 friends is an amazing value proposition.
The user auctions there fetch high enough prices that I would not hesitate to sell there as opposed to eBay. They also have their own marketplace where users can sell games at fixed prices, but the organic "hey I want to sell these, who wants to give me cash" lists of games[1] are quite successful.
[1] http://boardgamegeek.com/geeklist/66420/metalist-for-geeklis...