I still haven't bought any cards off the market. You can do that if you just want better players. Just make sure to create a buy order instead of purchasing them at asking price. If your offer is the current highest, someone will usually cave and just sell it to you for that price.
Flipping seems to be a bit harder this year. Last year, the tax for sellers was only 5%, and now it's doubled to 10%. This year, the only cards with a worthwhile profit margin are in the 15,000+ range. People are also wanting to complete collections for all the reward cards, so they're offering more stubs on their buy orders. Last year, a card would be selling for 7k, and you could be the top offer at 4k, and someone would sell it for that. Now people are making offers of 6,800 for a 7k card, so there are no profits to be had.
Starting with what I got out of my preorder packs, I've just been selling all doubles or higher value cards that I don't really need and then buying the 20 pack bundles, rinse and repeat. I'm trying to complete the entire NL East set, and to a lesser extent the entire NL collection, so most of my AL cards are listed as soon as I get them.
I'm not sure what my team overall is, I think it's around 84 right now. I haven't really added in any new players for a couple days. My starting pitching sucks, but that's not really a concern in the 3 inning games.