Good start with the MF. Seems like you have a good strategy with buys just like that!
FWIW - My greatest success in the last year has been buying sets on discount that are not overly talked about here on this forum, that on ebay seem to sell.
You mentioned the Tumbler, sure its good, but after fees etc selling most people are not making a ton of money...when it retires in a few years you may have a different story. That set has also been purchased in the thousands by members of this forum once it was released, Lego SAH said it would not be available until after Christmas, so people bought even more of it, low and behold it shows up at lego stores and online right before Christmas (The Lego group had fun with that set ).
With the tumbler, those who are hoarding huge quantities are reliant on people like you and other members buying up the rest of the inventory which will increase the demand by limiting supply, then those with large quantities of stock can sell huge volume at less profit per set and make money while you are left holding your sets wondering what to do.
Remember that while this site is very useful (I found that the friends vet and butterfly shop were great resellers and managed to find a ton of them at the local rural walmart), a lot of the people giving the advise are also 'stockholders' in the sets. They are also your competitors when selling online and sometimes regionally. Think about if we were talking about gold or other forms of stock.
You have two girls so you know very well the absurdness of the world of Frozen, it is a mega brand and its success is transient of many market factors, if you are like me you have paid over the top for Elsa dresses and dolls. Look what happened when the Frozen lego set was released early, two weeks before Christmas, first day sales of the set were lower to mid $100's then the market got completely flooded and after fees and shipping they were really only selling for a little more than retail (some will argue they bought at bulk and low cost etc). This was an example of where people looking for a quick flip, the majority here, got it completely wrong, they kept discounting sets so they could get the sale however there were such a relatively low supply, not many people understood that the success of Frozen and the value of that set was transient of any 'Lego selling' norms.
Have fun and enjoy, the friends sets are actually really cool and are very fun to build, In my opinion, which is probably worth no more than many others, that is the place for growth.