We want to buy only one image for using it as logo in our website. Can we buy it without having an acoount? The product that we buy can be used as a logo in timeless period. Also, after buying it, will anyone buy or use it again?
You can't buy logos on stock, even if the asset calls itself "logo". You never get exclusive rights to assets, so anyone who wishes can buy the same asset and use it as he or she wishes in accordance with the licensing terms. You have better to hire a designer creating an exclusive graphic for you. This will, however, be more expensive if the designer is working professionally.
If you want to buy assets without having a subscription, you can buy credits that can be used to acquire licences to assets. You could also take a subscription on the month, so that you can cancel that one for each end of the month.