Just joined this forum to reply to your post - it seems that with each buy/sell transaction, as an end user, you're risking losing money rather than profiting, especially if you bought it new, at retail price.
These are so close in spec that it would be advisable to keep your current model rMBP, as they hold their value longer than any other brand. Only choose to sell it if the software you have begins to run slowly or affect your productivity.
Keep it in a case/sleeve (live-in cases would be the safest bet, if it works for you) as much as you reasonably can to keep it looking new, and as long as you are respectful of the battery (no letting it completely die too many times) you should be a cut above everyone else when you go to sell it, and have no issue at all finding a buyer - especially since it will speed back up if you do a wipe/reset before shipping it out.
This is my input from a "conservation of funds" perspective.