First, create the additional usergroups you want at Admin Panel -> Members -> Manage Usergroups. Then select them as allowed registration usergroups at Admin Panel -> Members -> Member Settings.
At Admin Panel -> Themes -> Manage Templates, you can create custom templates for each of the special registration forms. Copy the regular registration template and modify it as desired.
Find the usergroup selector:
<tr>
<td class="labelscolumn"><span class="labels">{LANG_PROFILE_USERGROUP}:</span></td>
<td class="optionscolumn"><select name="usergroup">{USERGROUPOPTIONS}</select></td>
</tr>
Replace that with a hidden input field specifying the usergroup number applicable to that form: <td class="labelscolumn"><span class="labels">{LANG_PROFILE_USERGROUP}:</span></td>
<td class="optionscolumn"><select name="usergroup">{USERGROUPOPTIONS}</select></td>
</tr>
<input type="hidden" name="usergroup" value="6" >
for group #6 for example.You'll also need to add a couple more hidden input fields (anywhere, but let's say just above the submit button) to make the special template stick on an incomplete form submission reload:
<input type="hidden" name="TID" value="templatename" ><input type="hidden" name="custom" value="yes" >
Replace templatename with the name of the custom template.
Finally, replace the standard registration template with a list of links to the special registration templates:
<a href="register.php?custom=yes&TID=buyerregistration">Register in the buyers group</a>
or
<a href="register.php?custom=yes&TID=sellerregistration">Register in the sellers group</a>
That's all.