Я вот так всегда делаю.
RewriteEngine On
RewriteRule ^market/ - [L]
RewriteCond %{HTTP_HOST} ^market\.site\.ru$
RewriteCond %{DOCUMENT_ROOT}/market%{REQUEST_URI} -d
RewriteRule [^/]$ %{REQUEST_URI}/ [R,L]
RewriteCond %{HTTP_HOST} ^market\.site\.ru$
RewriteRule (.*) market/$1 [L,QSA]
RewriteRule ^market/ - [L]
RewriteCond %{HTTP_HOST} ^www\.market\.site\.ru$
RewriteCond %{DOCUMENT_ROOT}/market%{REQUEST_URI} -d
RewriteRule [^/]$ %{REQUEST_URI}/ [R,L]
RewriteCond %{HTTP_HOST} ^www\.market\.site\.ru$
RewriteRule (.*) market/$1 [L,QSA]
