Sell and buy prices are saved in a file named UserSavedPrices as the following:
a56a5112-a907-3acb-86b5-a238491b1a14:
'77401197': 123.0
'77401198': 111.0
USER UUID. ITEM BUY PRICE. ITEM SELL PRICE.
Then plugin will now "automatically" assign prices if they were assigned previously.