The short answer? Usually. Each event and venue may have different regulations but for the most part, if tickets are still available on SeatGeek, you should be a-okay to buy tickets for that event!
Some notable exceptions are Broadway and classical music productions which tend to have very strict door and start time policies, music festivals which require physical tickets or wristbands to be mailed to you beforehand, and some postseason sporting events like multi-session tournaments...etc.
In any case, if tickets are not able to be sold for a specific event which has already started, you'll receive an email and notification from SeatGeek letting you know that your order has not gone through.