I've had this issue before, and just replied to another guy with the same thing.
First here's what steam support recommends you to do:

"The error indicates that you are sending too many requests to the game's item servers. This most often happens when a browser extension is gathering information about items on a page, or when an IP address is shared across multiple accounts, which often happens when using a VPN, especially peer-to-peer VPNs.

Unfortunately, we have no way of removing these restrictions. We recommend removing any extensions that automate item and inventory activity and disabling all VPN usage. Please note there may be a delay of up to one day between removing the extensions and being able to browse the market or Steam inventories."

That was quoted from Steam Support^
Try those generic fixes, see if they work . If the steps above don't work, your IP from your ISP might be shared and someone else caused the block. First try to reboot your router, unplug your router from power for 5-10 minutes. This often causes your ISP to assign you a fresh, new IP address when it reconnects.
OR
This is what worked for me since nothing else did, use a different internet connection or mobile network just for the Steam Market place. Get the steam app on your phone and you can handle your market stuff on it. Hope this helps.