Any legitimate merchant who's traded in eastern Greenshade knows to avoid the old Merchant Tunnels.
In old times, when we first asked the Green to create a passage, the tunnels were an important byway, a defense against the wilder things that live in the forest.
Now, it's a haven for all that's illicit, illegal, and disgusting. Backhanded traders moving unreliable product at outrageous costs. Black market trade. Skooma. It's all there. A sure way to call one's legitimacy as a merchant into question is to pass through those tunnels.
For this reason, though the way is treacherous, it is advised to avoid the tunnels and take the low route over the bridge, beyond the bone pits.
Other good advice for Valenwood merchants is to travel well-armed, and to keep a watch at night. The Wood Elves of the area are known for their thieving, and some can be extremely unfriendly to strangers.
In general, it's a good idea to stick close to Woodhearth and other major settlements, rather than trying to deal with the backwoods Green Pact Bosmer personally. Let Wood Elves deal with Wood Elves.