@op, i will try to answer your simple questions one by one.
That is where my question lies. If the value is 1:1 then how much will I pay in bitcoins for example a desktop computer?
you can simply see the btc rates at preev.com at the time of purchase and then calculate the actual amount of btc that you have to pay with those rates. this is called 1:1. it means that you dont have to pay any extra btc to purchase the item.
Also how much bitcons will the merchant accept as payment?
merchant can accept any amount depending on your purchase.
If I pay 1 BTC, will the merchant accept it?
yes but dont send any amount. btw, almost all the merchants convert usd into btc with 1:1 rate so that you can directly pay the given amount.
How will we know if we use 1:1 as basis?
cross-verify the rates that your merchant is asking, with preev.com. if the conversion rates matches then it is ok, if it dont match then you should ask the seller about it. btw, you should also note that btc price always keeps fluctuating.