Overwrite=No,,"* Changing the setting to ?Overwrite=Yes? will have the added effect of deactivating all of your items from the website except for those listed on this datafeed. If this is not intended, keep ?Overwrite=No?.",,,,,,,,
Part #,Item #,Currency,MSRP,MAP,Checkout MAP,Selling Price,Inventory,Fulfillment Option,Shipping,Activation Mark
PS2-BEC-5780,9SIAFDG6V86915,USD,,0.00,False,26.98,37,Seller,free,True

Overwrite = Yes,,* Changing the setting to ?Overwrite=Yes? will have the added effect of deactivating all of your items from the website except for those listed on this datafeed. If this is not intended, keep ?Overwrite=No?.
Part #,Item #,Currency,MSRP,MAP,Checkout MAP,Selling Price,Inventory,Fulfillment Option,Shipping,Activation Mark
PS2-BEC-5780,,USD,,0.00,False,26.98,37,Seller,free,True

import pandas as pd

d1 = pd.read_csv(filename1, skiprows=1, usecols=['Part #','Item #'])
d2 = pd.read_csv(filename2, skiprows=1).drop('Item #',axis=1)

res = d2.merge(d1, on='Part #', how='left')

d1 = pd.read_csv(filename1, skiprows=1, usecols=['Part #','Item #'])
d2 = pd.read_csv(filename2, skiprows=1)

d2['Item #'] = d2['Part #'].map(d1.set_index('Part #')['Item #'])

In [106]: d2['Item #'] = d2['Part #'].map(d1.set_index('Part #')['Item #'])

In [107]: d2
Out[107]:
         Part #          Item # Currency  MSRP  MAP  Checkout MAP  Selling Price  Inventory Fulfillment Option Shipping
0  PS2-BEC-5780  9SIAFDG6V86915      USD   NaN  0.0         False          26.98         37             Seller     free

   Activation Mark
0             True

d2.to_csv(r'/path/to/file.csv', index=False)

#!/usr/bin/env python3
import pandas as pd

# use unique keys as index
df = pd.read_csv('1.csv', index_col=0, names=['key', 'value'], header=None)
print(df)

# read as series
keys = pd.read_csv('2.csv', squeeze=True, names=['key'])
print('nKeys:', *keys)
print('n', df.loc[keys])

value
key
a        1
b        2
c        3
d        4

Keys: b d b

      value
key
b        2
d        4
b        2