True, except that in my experiences, the lowest-end model usually loses the least over time.
I've been selling my macbook pros (sold over 15, yea I was THAT picky and young and stupid at the time) ranging from 13" to 17" since 2005 (well in 2005 they were powerbooks) on eBay and in my experience the amount you can get really depends on how presentable you sell your macbook pro.
For example, one person might fetch more with the 4GB model as to the 8GB model just because they uploaded better pictures, had a higher rating, put the item up for bid at a better time, loaded it up with more software, better condition, etc..
So money wise, its very very random or luck on how much you get for your machine because you could load it up with software and use all the tricks in the book to get more money out of it. This is also true with craigslist, it really depends on how much the buyer is willing to spend on your item.
Anyway but the one thing that IS consistent that I've found is that more people will be after your machine if it has better upgrades compared to all the same generation models. You can sell it much easier and on average will get a higher bid than say a 4GB machine in this case.
Sure the average eBayer is looking for the cheapest deal but the prices are all adjusted accordingly. The 4GB will still on average fetch a lower price than the 8GB model. For me I don't upgrade my machines for at least 3 years so to me the $200 extra that I'd spend on to enjoy the 8GB advantage is totally worth it.
Dont get me wrong, I'm still eying this deal because like I've mentioned before that it is a very very good deal even with the 4GB of RAM. But I know for at least myself that once I buy it and a few months pass by, I will get hit by the upgrade bug (usually on older MBPs I always upgraded the internals later on) and would regret it big time.
Also think about this, what if later on for whatever reason you'd want to upgrade the RAM because it wasn't sufficient anymore (I don't know new OSX runs better? Apps you suddenly use that you've never had to use, etc..)? This would make you upgrade a year earlier than expected. I know Macs have great resale values but man what a waste of money to dump the macbook air at a loss from what you've originally paid for when the next upgrade is just a minor spec bump just because of the lack of RAM.
For your usages it makes total sense to buy the cheapest since you use it lightly, not much a power user, and the fact that you guys upgrade every 2 years. But for me I don't upgrade so frequently because I think by upgrading so frequently, you're wasting more money no matter how cheap you originally got it for.
On a side note, I sold my 15" 2011 MBP on eBay a few weeks ago. I never had any issues with it and I had 16GB of RAM upgrade and it was more than fast enough for another 2-3 years. I sold it only because I thought that I'd get hit with the ATI GPU issue. I fetched more than the average because of the 16GB of RAM as to the other sellers.