As with most categories, the market for period frames is well off its high, which was reached in the early 2000's. That said, good frames can still bring money. Here are some things to look for if you are on the hunt.
First, a little history. The earliest frames were actually carved wood with gilding applied to them. Or, in the case of the Dutch, often it was a black finish. These early carved frames are valuable but also highly uncommon, so the odds fo finding one from the 16th or 17th century are pretty low, at least here in the US. For American frames, the highest dollar value frames typically will be those made by either a master frame maker like Stanford White or one of the master impressionists like Childe Hassam. These frames can bring north of $10,000 at auction. Suffice to say, they are pretty rare and not something you will likely find in a general re-sale or antiques store.
A little lower down the value scale will be the frames made by companies such as Newcomb-Macklin and Thanhardt-Burger. These are typically hand-carved and hand-gilded and can bring prices in the low thousands if a particularly nice frame. While these companies did make Louis-style frames, they are generally more associated with impressionist and craftsman-style frames. These frames, (and others like them by similar makers) you might very well find. Look either for labels on the back or hand-written frame maker's notes. These latter will usually reference size, color of base coat, and type of leaf used (22 gold leaf vs. metal leaf). Don't let the quality of the artwork deter you... more than once I have found exceptional frames with really bad paintings in them ;-)
Another frame company to watch for is House of Hedenryk. These frames often are Louis-style and will frequently have a pale wash. These frames were typically used for works by 20th century school of Paris artists, many of whom were carried by galleries in the US. Look for the Heydenryk label on the back of the frame, which ideally will still be in place. Thrift stores and resale stores are good hunting grounds for these frames since many people mistake them for later "shabby chic" frames. I have purchased Heydenryk frames for as little as $10 at re-sale stores and had them bring over $100 on eBay. Especially nice ones can bring several hundred dollars, so keep your eyes peeled!
Bryan H. Roberts is a professional appraiser in Sarasota, FL. He is a member of the Florida State Guardianship Association, the Sarasota County Aging Network (president), and is certified in the latest Uniform Standards of Appraisal Practice (USPAP) Equivalent