Greg-
You're almost certainly not going to have problems. As many have mentioned here, the citation you were concerned about is mostly about large mysterious transfers that are often a sign of illicit cash in the process of being laundered.
If you are selling equipment, you ought to create a receipt for the person you're selling the gear to with their name and contact info, keep documentation of the stuff (like the ads that you sold them with). I don't think you'll get much government hassle with this level of documentation. Now, if you were receiving payment in cash with informal receipts and no contact information, you're getting into the mysterious transactions territory where you don't want to be.
Nobody's mentioned that there may be tax implications for all this depending on whether music is reportable income for you. If music is your profession and your Alembic is part of your livelihood, then you were entitled to treat the cost of buying and maintaining your equipment as a business expense. When you sell the instrument you are supposed to report profits on the sale of your gear as business profits. If you had bought a $5000 bass and expensed it (used it to cancel out $5000 of business profits), then sold it for $3000, then the $3000 is business profit and taxable - unless you spend it on a new bass, etc. If you're liquidating a business and had expensed everything, it's possible you'll get questioned on this, but it will probably need to be a much greater level than $10 or $20K.
For civilians (people who don't make money playing their instruments, or hobbyists), the profit from selling your instrument is taxable as well, but I think somebody actually has to not lose money on a sale like this before anybody pays tax. :-) Actually, there are a lot of guys who sold their 70's Strat that they happened to hold on to for 25 years for a profit, but this is in the noise as far as the IRS is concerned. The CPAs may correct me here, but I believe that Uncle Sam would like a cut of a profit on a sale that you report, but if you take a loss (99.9%), it's all yours.
David Fung