I currently own a $50K note I want to sell at a discount to my Roth IRA, ok? What if I purchase a Note from someone else?

Can my Roth Flip investment properties?

I think you have a big problem with your idea. Let me play devils advocate. I want to put more money into my IRA, 'cause 2k is not enough! I buy an obscure bond for $100k, discount the price to $50k and buy it from cash in my IRA. A month later the Roth sells it for $100k. Do you see the problem?

I doubt if any custodian would allow this kind of transaction.

Yes, some custodians will allow you to acquire discounted notes from third parties... but not all.