I do not have a step by step guide for you on this but here is the general idea:
You set the session variable when the user logs in. You will check this session variable to determine the price to charge.
How you do that is to make sure you have a whole sale price in your table for your items. There should be a public price and the wholesale price.
Once you have that worked out you should just set it up to use the public price. Then you will need to manually edit the code on the page to have it use the wholesale price if the session variable is set to a particular value.
So if your session variable is called 'wholesaler' and has a value of 1 and the recordset on your product detail page was call rsProducts and the columns were 'price' and 'wholesale' you would update the code for the price in the add to cart server behavior like this:
$ATC_itemPrice = ($_SESSION['wholesaler'] == 1)?$row_rsProducts['wholesale']:$row_rsProducts['price'];
This type of selection statement to choose one value over another based on a condition is called a ternary expression. If the condition is true then the value after the question mark is used, if the condition is false then the value after the colon is used instead. So in this case if the session variable wholesaler holds a value of 1 then the wholesale price from the db will be used, otherwise use the regular price column.
This is only meant to be a high level overview, not a step by step guide. If you have any questions about any part of this please post back with your questions and I will do what I can to help clarify, or make other suggestions that may help.