it depends..
Holocluck, it depends on a few factors..
how quickly do you want to sell the land?
how much tier do you have personally?
If you own group land that you want to sell, you can either set it for sale and wait until someone buys it.. or you can sell it to yourself and then sell it.
Either way, after group land is sold.. the tier you donated to the group is still held by that group. so, you need to manually lower that before buying new land (unless you have tier to spare, in which case it wouldn't matter much). You manually adjust your donations in the Group Information window (Land & L$ tab), next to where it says "Your Contribution:". So, if you need all the tier you had donated for the land, change that number to "0".
If the land is sold while still owned by the group, then the proceeds will be distributed to everyone who has the "make/receive payments" permission enabled. If you want to keep all of the money, you either have to make sure that you're the only one with that permission.. or sell the land to yourself (for L$0), then sell it publicly.
As far as the 10% goes.. if the group has tier remaining, you can deed up to that amount of land at any time. (granted, you will need the tier to cover it when buying it.. it is an automatic calculation. so, if you buy land and deed it to the group 5 minutes later, you will still need the personal tier to cover it for those 5 minutes)
in short (to answer "How does it work if I wanted to sell land and get land somewhere else if I donated it all to a group?"

:
- sell land (to someone else, whether it's to yourself first or directly from group to anyone)
- lower tier (in Group information window, Land & L& tab, Your Contribution field)
- buy land (any amount up to and including the extra 10%, making sure you have tier to cover it before it's deeded to the group)
- deed land (with "Owner Makes Contribution..." checked if you're supplying all the tier for the group to own that land)
- do happy dance (unless something went horribly wrong)
hopefully that helps
