I would first recommend you see a lawyer in your state. It seems that you'll be selling used medical equipment. If so, that raises the question of whether or not you need FDA, State, or other Federal approval. There could be other legal issues and reporting requirements.
now let's say there are no legal hurdles to selling this stuff. Without knowing the state where the items will be sold we can't answer the sales tax question. every state is different. even in a state the rules can differ at the state, county, parish, and local level.
i would say you have commission income reportable on schedule C and subject to self-employment tax. the IRS takes a very broad view of what is a trade or business and for most trades or businesses the net income is subject to self-employment tax. here's the IRS viewpoint:
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.
