Why do people put $1 & OVC on aircraft bills of sale?
This practice is really more of an accounting/finance question than an aviation one, but it's common enough that it's probably worth addressing here.  

When reviewing an aircraft's registration record you will frequently see the price on the FAA Aircraft Bill of Sale form given as "\$1.00 &amp; OVC" (One dollar and Other Valuable Consideration) as shown on the example below (click to enlarge).

The "Other Valuable Consideration" can be anything of value: Personal favors, other property (a car, a different airplane), blackmail information on someone you don't like, or (most commonly) money in excess of \$1.  

In a strict cash sale recording the amount on the bill of sale in this manner protects the sale price from prying eyes: Anyone with \$10 to spend can request an aircraft records dump from the FAA which will contain every bill of sale filed, but if the price is listed as some small dollar amount plus "Other Valuable Consideration" it's impossible to determine the exact price paid using only the bill of sale.
Note that in cases other than a strict cash sale other documents filed with the FAA (for example recording a lien against the aircraft to secure a loan financing the purchase) may disclose the purchase price depending on how those documents are filed: Some include the amount of the debt the aircraft is securing, others simply state "The debt identified by loan number 12345." which would still protect the confidentiality of the sale price.



As a means to avoid taxation recording the sale price as "\$1 &amp; OVC" almost never works: The tax authority is going to ask you the value of the "Other Valuable Consideration" and base the tax due on that amount.
If you tell the tax authority the aircraft was really only worth $1, and record it as such, they're going to want to know why: Generally even an unairworthy aircraft is worth more than that for the value of its parts.

Conversely if you get a ridiculously good deal on an airplane it can be to your advantage to fill out the bill of sale accurately &amp; present that along with other documents pertinent to the sale to your tax authority: You may be able to get the sales/use tax based on the ridiculously good deal rather than an appraised market value.