Hello,
When you trade, you "consume" the buy wall. If there is no more buy offer versus 500$ (example), the value decline.
In a transaction, you don't trade your BTC for another currency, so there is no change in value. A transaction is a bit like a sell versus nothing. OFC if it's for buying goods, and if the seller will automatically sell the coins, you will help to reduce the value.
We can go deeper : As your transaction requires fees, and because you pay the fees, you will pay the miners. These miners can dump the coins they got in the fees in an exchange, so you can by this way reduce the value
But as transaction fees are really small, this doesn't change anything in fact.
