Do you know what method you used to make the card? Was it this one
, with the latest version of the installer (1.3)?
What version of CM7 did you install? If you don't know, check in Settings>About Tablet.
If it's 7.0.x stable signed, or a nightly earlier than 09252011, I would grab a nightly
more recent than 202. If you made the card with an earlier version of the installer, you'll need to update
that, too. Nightly 202 and onward set the default pixel density to a market compatible value.
You may still need to long-press your market icon, choose Application Info, and Clear Data (the button may stay gray for a minute, just hit it when it lights up).
A more temporary solution would be to download the LCD Density app, use it to set density 160 ("Killall" reboot option), then clear market data as above.