w/ comments

import requests
from bs4 import BeautifulSoup
import csv

def load_id_dict(): ## renamed to dict, more common abbreviation
    with open('strings.csv', 'r', newline='') as f:
        file_data = csv.reader(f, delimiter=',') ## renamed so we aren't reusing names (i.e. reader), even though technically they wouldn't conflict
        id_dic = {row[0]: (row[1]) for row in file_data}
    return id_dic

## moved headers outside loop, doesn't need to be initialized every time
headers = {
        "User-Agent": 'Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/78.0.3904.108 Safari/537.36'}

workingString = ""
for _, v in load_id_dict().items(): ## k isn't currently used, so use _ instead, but we still have the option to use it later if we want to
    URL = 'http://poe.trade/search/' + v ## removed redundant brackets

    page = requests.get(URL, headers=headers)
    soup = BeautifulSoup(page.content, 'html.parser')

    item_name = soup.findAll("tbody", {"id": "item-container-0"})
    item_prices = soup.findAll("span", {"class": "currency"})

    title = item_name[0]["data-name"] ## no need to loop over all the items, they should all
                                      ## have the same name, so just use the first one
    prices = []
    ## removed foundChaos
    for container in item_prices[:5]:
        price = container["title"] ## moved this statement to before the if
        if "chaos" in price: ## simplified
            price = price.replace(' chaos', '')
            price = int(price)
            prices.append(price)


    avgPrice = sum(prices) / len(prices) if prices else "N/A" ## make sure prices isn't empty

    ## use += instead of =
    ## workingString += title + "," + str(avgPrice) + "\n"
    ## but also use f-strings
    workingString += f"{title},{avgPrice}\n"


print(workingString)

filename = "out.csv"
with open(filename, "w") as output_file:
    output_file.write(workingString)