Monitor a web page for when a desired price is met
Constant reuse

Store your base URL, "https://www.example.com", in a constant for reuse.

HTTP codes

Some status codes, such as 200, are common and obvious, while others such as 429 are less so. Fundamentally these are all magic numbers, and requests.codes contains symbols - including too_many_requests.

URL construction

           allposts = [f'https://www.example.com&amp;Price={i["Price"]}'
                    for i in req.json()["data"]["Sellers"] if i["Price"] &lt;= 10]


Firstly - you don't need to be adding the URL in here. You can move the query parameter to the params dict to be passed to session.post. Then the URL can be passed to session.post directly without any baked-in query parameters.

Variable case

ConnectTimeoutError as E:


Don't capitalize E since it's a variable. Past that, you shouldn't even write as E, since you aren't using the exception.

Import long-form symbols

requests.urllib3.exceptions.ConnectTimeoutError


would benefit from an import of this class so that you can use it without package qualification.