This is how I would respond even if I didn't know this was a hypothetical situation.
Hi...Good for you for trying to work. I know from personal experience that living with a disability on a fixed income can be isolating and demoralizing. Work has helped me a lot physically, financially, and with my self-esteem. Unfortunately, Social Security disability benefits programs were designed on the premise that disability = inability to work. Now, with advances in medical care and assistive technology, many people with significant disabilities are indeed able to work, but we still have our disabilities, and may still need supports linked to our benefits.
For example, when I'm working (which has been most of the last 18 years), it's been fine that my SSI has been reduced down to $0. My earnings were more valuable than the SSI check. But because I'm a quadruplegic I cannot afford to give up my eligibility for SSI because it links me to Medicaid which pays for my respirator rental ($1800/mo.), wheelchair repairs ($1000 - $3000/yr.), co-payments for I.V. meds and nursing ($300/mo.) and 9 hrs. a day of personal assistance services ($3000/mo.). I explain how to keep the Medicaid without the SSI below.
In your situation there are two big issues.
1 - According to SSI eligibility rules, if you earn over the Substantial Gainful Activity (SGA) amount in the first 12 months that you are determined to be disabled, you are not "disabled" according to SSA and you will have to pay the money back. In 2006 SGA was $860/mo. and in 2007 SGA is $900. If there is any evidence in your bank account or elsewhere that you have additional income from Ebay, SSA will probably notice it, especially when you are required to present your bank statements at your annual Redetermination with SSA.
2 - If you remain qualified for SSI after the first 12 months, it's okay to earn above the SGA amount if you follow SSA's work incentive rules. If you have no other income besides SSI and your earned income, you can earn $85/mo. without affecting your SSI. For every dollar that you earn above that amount, your SSI will be reduced by 50 cents. If you earn enough to whittle your SSI down to $0, you may still maintain your Medicaid eligibility under a program called 1619b. This program calculates how much your disability-related expenses are and compares them to what any employer-sponsored or private insurance covers and how much your earned income can support. If your expenses are above what's covered, you can keep your Medicaid. These rules are more complicated than I've explained, so I strongly recommend that you read The Red Book and speak with a disability benefits advocate in your area to figure out how to "go legit." It's really a hassle to get caught lying to SSA, and it can haunt you later if you owe them money - They will take it out of your retirement benefits when the time comes.
See:
Red Book
http://www.ssa.gov/work/ResourcesToolkit/redbook_page.html
To find a benefits advocate, contact a local independent living ceter:
Directory of Centers
http://www.ilru.org/html/publications/directory/index.html
Good luck!
Alana
Benefits Advocate
Person with a Significant Disability