Alright, so I've taken advantage of a presale for Beyonce's Formation Tour in my town and snagged a couple pairs of sweet floor seats from Ticketmaster. I'm sure as hell not going.
How/when is the best possible time to sell these and for how much profit should I mark up for?
I spent $280 ea.
I go to concerts all the fucking time and I'd say you're actually better off selling them just a little while after the event (or just the floor) sells out, but way in advance of the date of the show.
As the concert date approaches, more and more people will be backing out (because muh flaky millennials always canceling on things) and so there will always be a big supply of tickets closer to the date of the show. On the actual day, you can often buy tickets for even cheaper than retail.
I bought a bunch of floor tickets to ASAP Rocky at $80/ea a few months back and sold them at $200/ea maybe 2-3 weeks after tickets went on sale (floor/general admission sold out within minutes but seats were available for a while).
Honestly there's no harm in trying to put up the tickets for sale earlier rather than later. You might find some cuck willing to spend $1000+ on a pair of floor seats sooner than you'd think.
You should consider things like availability of other shows in your area, popularity of Beyonce, population (to ensure a sellout and that it's a big event, etc.), and whatnot.
I rambled a bit but I'd start trying to sell them within the week desu.