If you sell for more than you paid (regardless of whether you sell your share to the other owners or to a third party) then you have a capital gain.
If you sell for one of the reasons listed in publication 523 on page 6, you may qualify to exclude part of your gain. If not, the gain is fully taxable.
https://www.irs.gov/pub/irs-pdf/p523.pdf
Also, if you sell in less than a year, then any gain that is taxable will be taxed as ordinary income. If you sell after 1 year, any gain that is taxable will be taxed at the lower long term capital gains rate.
