In fact, a large and elaborate garden may actually slightly impede the sales process as opposed to, say, a small garden with a few colorful perennials. Because so much of selling a home is about a buyer's vision for it, your sprawling garden might make it a trifle harder for some nongardeners to visualize an all-grass yard where their kids and dogs will frolic. Your typical garden-variety buyer might think "This will need too much maintenance" or even feel guilty about quietly harboring plans to rototill the bloomin' thing.
I should add that you might be inclined to nix a potentially good deal with such a less cultivating buyer, letting your emotions interfere with a profitable sale. If the garden has a lot of paved "hardscaping" areas and/or such components as a bridge-and-water feature, this could represent additional complexities to a buyer.
Not that the right party can't come along and jump at the chance to get wrapped up in that green oasis you created. In lot-constrained Great Britain, a survey by U.K.-based Halifax Estate Agents found that gardens are considered the "most important feature" by about 30 percent of home-hunters.
If your garden does become a selling point, make sure to be upfront about any plantings you will remove when you depart. You may, after all, feel attached to those "babies" you nurtured from infancy and not want to part with them. Hence, pot up any plants you want to retain before you show the house. But keep in mind that soil and lighting conditions at your new destination may be different (not to mention pests), so your effort might not be quite worth it, save perhaps for certain rare species that are costly to rebuy.
Know also that some gardeners feel great pain when they start deconstructing their creations prior to moving. You are obviously an accomplished, deeply rooted gardener who can deftly start with a blank slate at your new home. As Chance the Gardener in the classic movie "Being There" said, "As long as the roots are not severed, all is well. And all will be well in the garden." That would be your garden, wherever that may be.
Good luck and happy planting!