Betfair will always match bets at the best available odds at the time it reaches the market.
So if you try to back a selection/runner at lower odds than its current price
For example if you back at 1.50 and the price is currently 3.65 you will get matched at 3.65
Or
If you try to lay a selection/runner at higher odds than its current price
For example if you lay at 17.0 and the price is currently 6.8 you will get matched at 6.8
The only way you can back at lower odds OR lay at higher odds than the current price is to create a rule for a servant or automation file that will then place the bet for you when the desired price is reached.
This is done using a 'Fixed Odds Condition'
For example with a Back bet you might want to use ''Place at' fixed odds 1.54
You'd then add the following Fixed odds Condition
Selections last traded price is
< 1.55
OR
With a Lay bet you could use ''Place at' fixed odds 17.0
Fixed odds condition
Selections last traded price is
> 16.5
There is a automation file already made in the shared files library that backs as the odds shorten but it can be easily edited to lay as the odds rise instead, see link below
http://www.betangel.com/forum/viewtopic ... 43&t=11414
Alternatively, you could use the above and create a servant to do this, see the following link to download a ready-made servant
viewtopic.php?f=58&t=16235