PLEASE download for free this file, it is only 140k, and shows photos of some of the problem dials available on web based auction sites, and discusses why they cannot work.
LATITUDE ADJUSTABLE ARMILLARY/EQUATORIAL
There is a beautifully wrought armillary sundial for around $30 usd that looks nice, and in fact is very well engineered. However, the latitude index isn't even close to accurate for the armillary that rotates and should be calibrated by it. Looks nice, works if you use a protractor and plumbline, but it is not functional out of the box, not even close. There may be some good cheap ones, but at $30 usd - buyer beware!
LATITUDE ADJUSTABLE HORIZONTAL
Again, beautifully made, however in almost all cases the latitude index is so far off that it is useless. In one case the dial plate it tilted in the wrong direction.
There may be some good cheap ones, but at $30 usd - buyer beware!
LILY PAD DIALS, AND THE LIKE FOR GARDENS
Many can be made to work by calculating their design latitude, see here, and then tilted. However, some have the noon mark correct but the 6am and 6pm below or above the dial center, and they can never work.
There are some good cheap ones, just educate yourself.
CAUTIONARY NOTE
Web based auction dealers are as a rule an honest group, but they are not diallists. Be very wary of dials for sale on the web. Some are excellent, but they are not in the $30 range. If it is around $30 usd then educate yourself, see the notes above for the latitude adjustable dials that simply are incorrect. Some work fine, some cannot.
EDUCATE YOURSELF FOR FREE WITH SIMPLE SHADOWS.
READ THE BUYERS GUIDES there are three of them
Mike Shaw's Carl Sabanski's Simon's (has pictures of some incorrect dials and of some that are perfectly usable.
Study those buyer's guides. Study closely the dial you want and ask what guarantee exists for its accuracy.
A guarantee is only as good as the seller's reputation.
When you get the dial, check the reverse engineering link to verify your dial, and return it asap if it cannot work. This spreadsheet helps in the reverse engineering process mentioned in the above notes.
Check the NASS and the BSS web sites (see main page) for ideas and with reputable diallists.
Educate yourself. Download the SIMPLE SHADOWS book for free.
YOU ALWAYS GET WHAT YOU PAY FOR WHEN IT COMES TO PURCHASING A SUN DIAL.