Working with BTC is hard for newbies because fees are unknown until the end. If we want to buy something that costs $450usd all the fees have to be calculated in advance.
I assume you want to buy an item that will cost you $450 in BTC, so you need to buy BTC first
it would be easier to (make) deal with the item seller in term of bitcoin amount, instead of $ amount
so you can lock in the amount of bitcoin and not be tied to value in $ anymore
I found an offer on wall of coins, for example....I asked for $450, got an offer for .06618674 btc which comes to $397.15....im $53 short. This does not include the miners fee which they take out at the end, causing another shortage. Lastly, we have to transfer USD to the seller, which is complicated usually requiring a trip to a bank to do a moneygram or western onion. Any rule of thumbs to help in calculations?
this is what I would call buy-sell price spread, the seller quotes a different exchange rate than you expected
in your case you were using a rate of ~$6000 * 0.06618674 btc = ~$397
while the seller was using a rate of ~$6800 * 0.06618674 btc = ~$450
so basically you've bought btc at a premium price of around +13%
note: all these are under the assumption that on may 9th the price of bitcoin is at ~$6000