Hello and welcome to the Harbour Lights (HL) forums,
Selling HL lighthouse replica's is a challenge these days, as Daniel alludes to in his post. The market is not nearly as active and vibrant as it was 20 or so years ago. There are many reasons / theories why, but that is a for another discussion.
You said your mother-in-law has a large collection of lighthouses and lighthouse pictures. They are Harbour :Lights replicas? Your post didn't say specifically what brand of lighthouse replicas you have. Some of the responses you receive here on the HL forums may not be helpful if your replicas are from a company other than Harbour Lights.
Some of the (relatively) better deals for HLs on ebay are actually for the HLs Christmas Ornament pieces. These are much smaller than HLs Limited Edition replicas. Other ebay choices are HLs Little Light of Mine line. These are sized in between ornaments and Limited Editions. As you are poking around ebay, try to compare apples to apples to get the best idea of the values that your pieces may have.
Years ago, before the actual end of Harbour Lights, the company, the secondary market was already beginning to fade. I had many HL replicas that I could no longer display and found that a willing buyer was hard to come by. One way for me to find another home for some of my pieces was to donate them to lighthouse preservation groups to be used as fundraisers for their organization. I gave several pieces to the Florida Lighthouse Association and others to a couple of groups working on preserving their local lighthouse.
Admittedly, this won't raise any cash for your mother-in-law, but it may not cost you much, if anything, if organizations like that are still willing to accept the replicas from you. At least your pieces would help preserve a lighthouse or two from the many that your mother-in-law enjoyed over the years.
Best of luck to you!
Weeki Wachee, FL