problem with database access in same time

SQL Server won't solve this, but gives you the opportunity and the tools to react properly.
In general when you decrease the amount of the pens, you issue a single update statement. This is atomic, but can happen that the two users issue this update so that it is executed one right after the other. In that case If bpth are trying to buy those 2 you will end up with -2, which is bad.

Let me draft two approaches (the amount you want to reserve is 2 in both cases):
A) Optimistic approach a little bit modified
1) The client issues UPDATE store SET amount = amount - 2 WHERE product=123456 AND amount>2
2) ExecuteNonQuery returns the number of affected rows. If in that instant the amount is less than the desired value, the statement won't be effective because of the last condition. So it will return 0. You can react and notify the user.

B) Pre-lock
1) Wrap following statements in a READ COMMITTED transaction
2) Issue SET LOCK_TIMEOUT to set a short timeout
3) Issue an UPDATE store WITH (ROWLOCK) SET amount = amount WHERE product=123456 AND amount>2
4) Check the affected rows. If it is 0 than you don't have enough pens to sell, and you can ROLLBACK immediatelly, and alert the user. In this case, no row will be locked!
5) At this point any other user trying to update the same row using this approach will fail after the timeout specified before, as the row is locked.
5) Do other things.
6) Issue the regular UPDATE to decrease the amount.
7) COMMIT transaction
This second apporoach is interesting in real-world situations where the user is not buying a single item at once, but is preparing a complex order. Actually this approach is locking out the other user from that specific product. So you better not keep it locked for too long. In such a situation it is better to add an other field (let's call it reservation), and manipulate that field during this reservation process, instead of decreasing the actual amount. The amount is than decreased when the order is finished, or when it is actually shipped.

A simple solution is to show the data consistently, if you're having 3 pens, then show 3. But let the first user who gets to order the 2 pens would get a chance to place the order, the second one if unluckily misses the opportunity by an instance of time, then show him the error message, or ask him to contact the shop owner for order etc. This is because, you can only provide 3 pens at maximum, the first one gets 2 and the second one can get (at maximum) only one pen. No matter what he does, or how ever he gets to click on the buy he cannot purchase 2 pens then. So you can just simply show him a message, saying, items are sold. There is no problem to be guilty for telling your clients that someone else bought the items he is looking for.

This is a very basic problem in database systems, where the users get to view the same data from the database but what do to when there are two members to order for same quantity and so on. The thing is, to make sure that your data is consistent, and every user is getting the live data from the database, once that has been taken care of, the only problem left is a delay in a few seconds before first user clicks for the order. Then you can show the second user an error message, saying, the items are sold. He will know that he is late.