My sister has also been looking for a sectional. We did a HARD shop for 3 days of all local furniture stores in our area (50 mile radius). I was most impressed with Hallagan furniture. Least complaints online. Expensive, about 7-8K but that's what i would have bought had I been in the market. She chose a less expensive Bernhardt that was made in China-$4500 leather, not the fake leather.

I look for manufacturers that are made in the U.S.A. and offer a lifetime warrantly on frame and suspension - that is usually a good clue it is made well. 8-way hand tied springs are the gold standard, and sofas that feature that usually have quality construction throughout the piece. Some manufacturers do good sinuous springs, but some do not, so it's a little more iffy. I'm partial to Lee Industries and The MT Company, but also love Carolina Custom Leather for leather pieces. These are what I sell so am most familiar with them. A seat cushion with innersprings will hold up the best. Avoid down unlessy you are okay with flipping and fluffing daily.

Some other manufacturers with good construction are:

Hancock and Moore

Taylor King

Stanford

CR Laine (though I have read some reviews that their quality is suffering lately)

Thayer Coggin

I've personally had a Lee Industries sofa for 15 years that has been through a ton of moves and it is still great! All I've had to do is replace the slipcover. I also got new cushion cores, though I didn't need them, I just wanted to switch to something firmer.

You can find a retailer near you that can help from the manufacturers websites, but if you can't find one near you, let me know and I can help!