Can I deduct self-employed expenses

Short answer is "Yes."

You are obligated to report any income -- including that from self-employment -- whether you received a 1099 or not. Now, going to a convention in Vegas has some ancillary benefits (not overlooked, I'm sure, by the marketing company who beguiled you into their program). So, I would not attempt to expense the entire trip and report a loss, since your actual income was only $138. You won't owe SE taxes anyway, since your net is well under the $400+ number ... but it appears you might actually qualify for the Earned Income Credit, and any self-employment income will feed into that calculation. (Of course, you may have extensive investment income which would disqualify you from EIC, or age also might do so.)

I would suggest you consider filing a Schedule C-EZ, deduct an arbitrary $100 of the "convention travel cost" (figuring 87% of that trip was for fun) ... and you'll wind up with an insignificant -- but reasonably reported -- net income of $38.

Bill

Why be so conservative? If the conference was 4 days at 8 hours a day, and you were in Vegas 4 days, they why can't you deduct the entire airfare and all your meals (provided none of them were extravagant)?

Perhaps the benefit of being conservative might be to avoid raising the eyebrows of anyone at the IRS who might be skeptical of the _business purpose_ of this apparently one-shot enterprise, when the outcome was a loss that's being taken against ordinary income.

If you have an ongoing business that has a clear intent to generate a profit (preferably producing income in at least 2 out of 5 years), that's another matter -- but I suspect you wouldn't be asking this question of that were the case.