Mostly buy/sell orders are used to trade stuff while you are not online or to make offers that others can fulfill. For every trade someone has to place an order. You can use instant trades to fulfill orders that others have placed or you can place orders so that others can fulfill them.
The advantage of instant trades is that you avoid the cost of the broker fee. The advantage of placing the order yourself is that you can set the price.
As for skill Marketing
is a must so that you can sell stuff (either instant trade or sell orders) in stations other than the one you are docked in. Accounting
is useful to improve your profits when margins are tight. Broker Relations
is only good if you are placing orders instead of instant trades, and again helps most when margins are tight. Trade
both increase the number of active orders that you can have and are requirements for many of the other skills. Procurement
are only used when placing buy orders at distant stations; can be helpful in that you can avoid flying to a distant station just to place an order only to fly back again after.
Good luck with your trading.