I don't think that the MySQL traffic will be overrun with updating a value when it's selected. I think you will need to add a few things because not only do you need to update the stock, if that's what you change, but you need to see if a user session has timed out.
So if you have three tables: Stock, Cart and User...
Stock columns: item_id, item_name, item_quantity
Cart columns: cart_id, user_id, item_id, item_quantity
User columns: user_id, user_name, last_activity
This means that everytime a user clicks on something (navigates) you need to update their last activity witha time stamp. So if a user doesn't move for say 30mins, then you can free whatever he has in his cart.
Now the tricky part is how often, or how you want to check if their activity has been longer than say 30mins. You could run a cron job every hour, or 30 mins which looks for any expired sessions and systematically checks carts to see if the user is still logged in (check activity has not expired) with some table JOINS and then release that stock.
You are probably thinking that this is a longa nd resource consuming way about it, and it could be if you run your crons *too* often, but the major problem is when a browser is closed it does not send a note to all the website it's visited to say, "I'm logging off", so the only real way to do it is with databases or sessions.
Having said that, there could be a way to attach a session_id to a the cart to avoid having to store last activity in the database, and simply check if the session is still considered active, or still exists. I haven;t done this before, so I can't comment, but I imagine it has it's own flaws as well.
MySQL is very good, so don't be scared to use it for resource reasons. You could cause issues, but a couple MySQL queries will not break the bank.
If you had a cron job running every 5mins, updating 10,000 rows 6 times, then we might advise a different route, but just try use it and if it's too resource consuming then think of a different way. Most things can be optimized, but you have to start somewhere.
