Selling on Audiogon

Do we have a resident accountant on this? I already pay taxes on 96 things. If I sell interconnects to a buyer in Montana and deliver them do I get to deduct travel from MA? Only if they're balanced?

Does this mean if a buy an amp new for $10,000 and pay the $900 state tax and then sell it used for $7,000 that I now must pay income tax on the $7000 sale? Some 20-30% of additional tax beyond the initial tax paid? 
 

Double taxation?

I sell a used table we no longer need on our local Buy, Sell, Trade Facebook neighborhood site for $600 and now must pay federal income on this sale? We already paid the tax sales tax on the table 10 years ago and sold the darn thing for 1/10 of what I paid. Now I pay income on this yard sale type of sale. 
 

This seems so wrong.  Just me? 

Not a tax accountant but I would suspect if you keep good records and can show what you initially paid including taxes then you would just that initial amt from the current selling price and owe the IRS only based on the difference. Still don't agree with this but this would make your tax liability a lot less. Hopefully a tax accountant will chime in.