"markered" rugs are exactly one would expect - carpets colored with a standard permanent marker to hide faded or otherwise worn areas.
When done on the front of the carpet, the intention is often to avoid costly repairs. Carpets may be markered because the repaired value may be considerably less than the cost of replacing the rug. While not an uncommon practice on the back of a rug, markering on the face of he rug is generally not well regarded by reputable rug dealers. Markered rugs are often deemed un-washable as bleeding often occurs.
The photograph below shows the front of an antique kerman from the fourth quarter of the 19th century The carpet has heavy wear, and was markered at one point. All arrows below identify areas where foundation/warp shows. The purple arrow points to a white string (warp) running vertically in the rug. The green arrows show the foundation as well, however these are markered areas which disguise wear and were originally white.
Below is a photograph of the same rug in a different area. Look closely at the green arrows. They point to areas of the foundation which have been markered and would otherwise been white. Notice how the marker is faded, and nowhere near as deep as the adjacent red dyed wool knots.
Below are carpet markers.
See more info on markered wefts