You have two kinds of buys and two kinds of sells: market/limit buy/sell.
Someone has L$10k and puts that up for sale as a limit sell, which means they pick the exchange rate they want their L$ to sell for, namely L$266/$1.
That limit sell order will just sit there, waiting for someone to come along, interested in buying (for simplicity's sake, assume there's just that one order on the whole LindeX

).
You come along and buy L$6k worth, which by default is a market buy, meaning you're willing to pay whatever exchange rate that are open orders for. In this case L$266/1.
The seller gets your US$, and you get L$6k, leaving L$4k in the seller's order.
Now imagine that a second seller came along, and put up L$5k up for sale at L$267/$1, undercutting the existing seller. When you do your normal, market buy order, you'd get L$5k worth at L$267/1 and L$1k worth at L$266/1.
A market order is always directly filled by available limit orders in order of best deal to you (so for buying that means from high exchange rate to low, for selling it means from low to high).
If you want the maximum amount of L$ for your $, then you place a limit buy order, which means that you pick the exchange rate you'll get for your order, rather than be stuck with the market rate which favours sellers, not buyers. Depending on what rate you pick to buy at, your order will sit there for a few seconds or minutes, to days, or forever.
Step-by-step:
https://secondlife.com/account/currency_settings.php : change LindeX settings to advanced
https://secondlife.com/account/manage_balance.php : put some US$ on your balance (minimum $25)
https://secondlife.com/currency/buy.php : scroll down to "Open Buy Orders at the Best 20 Rates ". For the last few months it's always been a few million stuck at L$277/$1, so if you want to get a quick buy, scroll back up.
In the limit buy: Quantity - 6817 ($25 worth, adjust as needed); Exchange Rate - 277 and click buy
Your order should fill in a few minutes and comparing to a market buy you would have gotten 6570, or about L$247 less. It's up to you whether that's enough to bother with or not

.