What your intent is is irrelevant; income from a "hobby" is still subject to tax, as Self Employment (SE) income. SE tax is levied if your annual SE income exceeds $400.
SE tax is
over & above the regular income tax owed on such earnings; it is the Social Security tax, both employee & employer's portion, currently at 15.3% combined.
If your earnings from the resale of domains is large enough, the registrar that you are reselling for will/should issue you a 1099; in this case, the IRS will be expecting to see you report such on your returns. If you receive no 1099, and fail to report such earnings, you do so at your own risk.
See
Self-Employment Tax