For the UI, would you recommend disabling any selections that conflict with the ones already in the betslip based on the SessionCreate response?

Or would it be better to keep all markets active (as per feed), allow users to add any selection to the betslip, and then validate everything again during SessionCreate, showing any eligibility or conflict errors at that point?

For certain, no selections that are not among the available markets should be allowed on the bet slip. The way we created our demo UI, we have a switch into the "bet builder mode" where only the markets from Available Markets are displayed, and the selections available to be put into a bet builder bet are clearly indicated. Their individual odds are changing into the total odds of the bet builder bet if that selection was selected.

Each time a user adds another selection, the available markets will get sent. The markets which are not available anymore should be suspended. When a user is constructing a bet builder multi-selection on the bet slip, every time they add one selection, there are fewer markets/selections available for them.