I am considering purchasing (with trade credit from DigitalEyes) the light up Greatest American Hero tin, the Martians & Medicine Mash set, and the Alias Vivaldi Box. I am not sure, however, that I will want to keep these after watching them. Typically, how quickly do special "limited" edition sets like these lose their value? Thank You.

IMHO DVDs should never be looked upon as an "investment". Additionally if you do purchase these w/ this in mind, things generally hold their value more if they are still sealed/unwatched.

They'll lose their value right after you open them. The only "investment" dvd's I'd ever buy would be out of print criterions at a low price.

I say if you don't plan on keeping them, use netflix.

Buy movies you want because you want to WATCH them. Invest your money in the stock market, or a CD (and not the round silver kind).

This advice also goes out to all of the 30 year olds who are waiting to cash in their blisterpack'd Star Wars Episode I figurines to pad their retirement.

There's not really a definite answer. Like any collectible, it's just supply and demand and is subject to whatever someone is willing to pay. Sealed sets command a higher price and are easier to sell because the buyer is less worried about damaged/missing material, but you can still sell open sets. There's a risk since there's no guarantee that you'll get your original purchase price back, but we're not talking about a substantial amount of money here. Remember that technically, all DVDs are limited editions. When they stop selling, they will stop manufacturing them. Anything that's popular will likely have a reissue eventually and even if it doesn't have all the features, it will devalue the previous limited sets, so don't get greedy if it does go up.
If a limited set doesn't sell out quickly, chances are that you won't break even, but at least you'll have a set of DVDs to watch. If you won't miss the money, then go for it.

Why don't you just Netflix it? You won't have to worry about resale value, and it definitely would be cheaper for you.

Buying DVDs that are still in-print to resell is generally a bad idea.