Your contract makes it clear: no other "occupation or business". Having a garage sale is not a business; even the IRS says so. Selling regularly on Amazon is likely to be a business; selling one used item is not.
The IRS says, at http://www.irs.gov/Charities-&-Non-Profits/Trade-or-Business--Defined
"Trade or Business" Defined
The term trade or business generally includes any activity carried on
for the production of income from selling goods or performing
services. It is not limited to integrated aggregates of assets,
activities, and goodwill that comprise businesses for purposes of
certain other provisions of the Internal Revenue Code. Activities of
producing or distributing goods or performing services from which
gross income is derived do not lose their identity as trades or
businesses merely because they are carried on within a larger
framework of other activities that may, or may not, be related to the
organization's exempt purposes.
The intent of your contract is to keep your work efforts solely focused on your job, even if it's off-hours. That way, there is no question that anything you develop, during that period of employment, no matter what day or time it may have been done, belongs to the company.
You can volunteer, part-time, elsewhere, as long as you aren't given a regular job role. You can play sports and even be captain of the team, as long as you're not paid for it and you don't hold a business-related position in the organization.
It's not all that fuzzy. If you want to sell your hobbycrafts on the side, you're breaking your contract as that is a side business. It's not about how -small- the business is, it's that you've agreed that you will have only one job, one occupation, and work for only one business. Period. And even if others get away with breaking their contract, you still would be risking your job if you do the same, and violating the terms to which you agreed.
Here is an example from some slightly clearer language in a standard contract; it allows one area of exception that your contract may not spell out:
The EMPLOYEE shall devote EMPLOYEE’s full business time and best
efforts and abilities to the COMPANY in the performance of EMPLOYEE’s
duties and responsibilities determined and assigned by the COMPANY’s
_______ and shall engage in no other business, either directly or
indirectly, in EMPLOYEE’s own behalf, or as an employee or
independent contractor of another employer.
Notwithstanding the above, the EMPLOYEE shall be permitted, to the
extent such activities do not interfere with the performance of
her/his duties and responsibilities to (i) manage her/his personal,
financial and legal affairs, (ii) serve on civic or charitable boards
or committees, (iii) serve on boards of other companies of
organization that do not compete with COMPANY or any related entity in
any shape or form, and the EMPLOYEE shall be entitled to receive and
retain all remuneration received by her/him from the items listed in
clauses (i) through (iii) of this paragraph.
Another way to look at it is to consider whether that activity might be something you'd want treated as a business if it's profitable. You can't deduct your hobbycrafts costs if it's just a hobby, despite being required to report the income; if you want to deduct the costs from the income, then you have to define it as a business. The same applies for those selling on Amazon: to deduct the costs of doing business from the income produced, you must file the very form that acknowledges it's a business. When working for others (such as a non-profit) as a volunteer, it becomes an "occupation" if you have identified responsibilities, a consistent role, or any of many other aspects that show that you function as regular employee (even if unpaid and part-time.)
But when in doubt: Ask the other party. They might even give you permission to do something outside of the contract terms.