When deciding which cards to sell and in preparation for a show, stock up on the local teams of all sports of all years, especially the most recent. Don't forget the team cards, especially the football team cards with rookie players on them. You can sell them with the rookie card and offer a discount if bought together.
Now is the time to create your wish list on cardtarget before shopping. If you have time, set up buy orders at the price points you're willing to pay. Trade away cards you don't think will sell at the show on the trading post for ones that are more of a sure-bet for your area. See if you can get a good deal on an in-hand from ebay to save on shipping fees. (This is when it's advantageous to have a lot of reward points in your account so you can save on shipping fees.) Even post your in-hand wants on the etopps message board with your asking prices.
When it comes to quantity of a player, don't go nuts on a hot player. If this is your first show, you have no idea yet what will sell. I wouldn't order more than 3 of any card.
If three of one title sells, you can sell to the next person who wants it by taking down their email and mailing address and shipping it to them at a later time. By accepting their payment for the card at the show, you're already on your way to starting mail orders. I'll discuss more about this in future posting(s).
From my experiences in selling at card shows, the sport that is in season usually is what will sell the most. And football. You can sell football any time of year. If the card show is focused on baseball or football, follow the trend and stock up more on this sport than other sports.
Another factor for determining which cards to sell can be based on who will be signing at the card show. If you have a card for someone who is signing, you should have at least one of their etopps cards in case someone is looking for a something to have the player sign. I've seen this happen a good number of times but it will not happen every time.
You also want to gravitate towards the teams and players who are performing well this season or did well last season.
You also want to stock up on a lot of Classics. People love these cards. I've seen one person at a show criticize etopps cards and wouldn't stop blabbering until he saw the Thurman Munson classic card. He shifted gears and started gushing over how beautiful the card looked.
If people collect for the history of a sport, you have to have classics for your area and beyond, especially Yankees. Since etopps has a lot of Yankees titles, it will look good to have Babe Ruth, Lou Gehrig, Mickey Mantle, Munson, Roger Maris and Mattingly all lined up in a row. You'll be surprised how much a row of classic etopps cards can do for your table.