Spoiler alert: if you are in an events business, "We emailed a ticket" does not constitute delivery of the purchase. Successful conclusion of the event constitutes delivery. Most events businesses structurally need to pre-sell tickets. They will, virtually inevitably, have more issues with collecting payments than e.g. a software company which says "You give us a credit card we give you a working download link." This is not a result of caprice by Paypal -- more than 1% of scheduled events fail to actually happen; when that happens, Paypal gets hit with hundreds of chargebacks.
More details: https://www.paypal.com/webapps/mpp/brc/presale-policy-and-re...
Events businesses: https://www.paypal.com/webapps/mpp/brc/account-reserves-tick...