Since no-one wants to start a discussion, I'll just argue with myself

Ad. 1) Since people want to get rid of some extensions they already tried I wouldn't give NICs spent on that extension back.

Ad. 2) I would like to see that kind of system. It would help optimizing your current extensions.

Ad. 3) I wouldn't allow people to downgrade this extensions like Marketing/purchasing below the level of how many offers you have on a market atm, i.e.: You have 10 sells on a market and basic marketing lvl 5 and advanced marketing lvl 1, you can downgrade basic marketing by one lvl to lvl 4 but you can't do it for advanced marketing since your maximum orders will be 6 and you still have 10 sell orders on a market.

Do you have any other ideas/suggestions?

When there's no one in the corp channel to harass, I sometimes argue with myself.  Sadly, I usually lose those arguments.

Anyway, I'm in agreement with you, but my opinion really only extends to the first item.  NIC should not be returned after you take an extension to 0.  In fact, it's my opinion that one of the costs of shuffling EP around should be having to purchase extensions at their normal price a second time if you should drop one to 0 and then have the whim to raise it again later.  It'd be a reasonable additional NIC sink...we could use just a few more of those around here.