So, a few observations...
First, you say that you LOANED some of the purchase price.. Are you going to get your loan paid back prior to profit split? Or are you going to ignore the loan and just get 50% of the deal?
Are you going to forgo the commissions or are you taking commission and THEN 50% of the profit?
Did you spell all this out prior to your deal? Do you have any type of written agreement?
In theory, you should file a partnership return with the appropriate Schedules K-1. These amounts will be reported on your respective individual Forms 1040.
I don't suggest that you modify your listing agreement and take your 50% as commission. Your share of the profits is just that. A 50% share of the profits - it's not a sales commission.
Finally, I'm not sure who gave you the idea that "profit from the occasional flip" is not subject to self-employment tax. That's just not correct. Income from any property that you purchase to wholesale or rehab and sale is ordinary income and subject to self-employment tax. That said, the cap for income subject to the tax is around $117,000 - so if your W-2 income is in excess of that amount you will avoid self-employment tax on these profits for that reason.
I strongly suggest that you run this by your own tax pro. He (or she) is in a position to know your particular tax situation and best advise you on the structure of the deal.