@topdom,
1) Those 3 letter .co domains you dropped, if not picked up before the drop, will be retained by the registry as premium domains. It's their policy. I generally renew my 3L .co domains.
2) I think Afternic's reseller network is better than Sedo's. Although many of the resellers are the same.
3) You should make the decision to use Efty. They have one of the best offerings, IMHO. But I'm not clear how much kudos they have with the buyers. They might prefer a fast transfer instead. I would, and I do as a seller also. Obviously prices are higher with a fast transfer though. Which might be a consideration. Endusers, as a whole prefer things simple and straightforward, generally, with the domain appearing at their choice of registrar with no fuss, no muss. And you cannot blame them for not wanting to jump thru hoops.
4) There is no real difference between your own website and Efty, except maybe Efty looks nicer
Both should be going thru Escrow for the security of the buyer and seller. They both have a disadvantage over fast transfer in that you and the buyer, are left to do all the transfering manually. Which is totally automated with a fast transfer.
5) Outbound is not a must. It depends on your business model. If you have outbounded all 1000 domains (all credit to you) and sold 10. It's probably time to drop those 990 domains. Unless 1 or 2 take your fancy to renew.
IMHO. Fast Transfer is the future. It makes everything very easy for both the buyer and seller. I'm going headlong into fast transfer this year. I have been reluctant to set fixed prices in the past. But I'm fixing my prices plus 25% (=20% commission on sale), as fast as I can (with all my other demands on my time). My website prices will be 10% cheaper than Fast Transfer prices, for the bargain hunter. All domains point to my website. Not to Afternic/Sedo. It's going to be a lot of thought and hard work to get this setup nicely.