Just so you know, the following is strictly my opinion!
If you have a choice between a regular serial number, and a "special" serial number (such as a radar number or repeater) it is definitely worthwhile to get the "special" serial number. As far as I can see, there is at least some interest in banknotes with special serial numbers, though I would argue that for the most part it is only slightly significant. I think that Asian collectors are big fans of "solid" serial numbers, such as 222222, though I've noticed they prefer solid 7's and solid 8's.
As for selling your collection, I always recommend eBay. It would be difficult to sell an entire collection outside of eBay or another specialised auction house. Selling Euros is particularly difficult, I believe, because they are not particularly attractive banknotes, they are fairly common, are quite expensive, and their forecast value for the near future is not looking good.
I hope that helps. Cheers.