Bob's take on what the IRS does is exactly correct; they compare paperwork from different sources and flag discrepancies. The letter is just an inquiry, not a final decision. You may need to amend your return to cover it. If your tax preparer had the 1099 and messed up they should amend for free and cover the penalties and interest, if any. If you didn't give the 1099 to the preparer, the cost and interest and any penalties are on you: garbage in, garbage out.
I think the IRS is doing more of this sort of paperwork inquiry/assessment than ever before for individual taxpayers. I got one from the IRS for a recent tax year because the IRS didn't have a W2 and W3 for me but had my 1040 showing the income. I got a letter proposing a drastic increase in my taxes because the IRS assumed that my
entire salary was untaxed when I'd actually been fully withheld and paid in. I got it straightened out eventually and the IRS rescinded the entire proposed assessment. Scared the heck out of my wife but having handled disputes with the IRS for clients, I understood the nature of the inquiry and how to straighten it out.
In other words, don't panic.
You may want to look at this page on the IRS web site, which explains the rules on capital gains taxation of collectibles:
https://www.irs.gov/taxtopics/tc409.html