Many issues here and difficult to go back and forth with questions:
Selling equipment to a business that you are not associated with is different than selling to a business that you own. The IRS is always very wary of "non-arms length" transactions that occur with the later.
If this is not a hobby as you initially stated, is this going to be a trade or business in which you are actively engaged in making money?
Why would you want to sell equipment to your LLC, which in fact is you, and recognize gain and pay tax? You will need to be able to determine the FMV of whatever you are selling and support this if audited by the IRS. I can't imagine what you would be selling that would create a gain, and therefore, why would you not just contribute at your cost basis? You cannot recognize a loss on a related party transaction.
You may want to discuss with a tax professional to get you started down the right path. As I said previously, there are many issues and different tax consequences that could have long term impact on you.
*A reminder that posts in a forum such as this do not constitute tax advice.
Also keep in mind the date of replies, as tax law changes.
