Hard to say why the seller drew attention to the frame, but s/he is being upfront about the
weight, which is a good sign. Checking the frame carefully would be an important part of the pre-purchase inspection of any trailer this age. Pay special attention to the main rails just under the front cabin around where they bend to form the A-frame, especially on the driver's side. Some issues can be repaired without a full frame rebuild/replacement.
That and other Boler-specific issues to look for can be found in
Ian G's Boler Guide. You may want to print and use the other document in the link also, a generic pre-purchase checklist.
The
Boler in the linked ad appears to be sitting a bit higher than stock. That probably means it's had a new
axle, not unusual given the age. The seller doesn't mention it, so it was probably done by a previous owner, but it'd be worth asking. That's a plus if done right. I'd check the installation while you're looking at the frame.
Brakes aren't mentioned, so check to see if the
axle has a square mounting flange behind the wheel. That would allow you to add them.
Other miscellaneous observations... Ball size is incorrect in the ad, probably 1-7/8"- seller may be confusing it with the receiver opening, which could be 1-1/4" Class I on a Kia Forte. It has a 120V dorm
fridge in place of the original icebox, a drawback since it only works if you have power. No
furnace. Cabinet doors look nice, and the interior appears generally clean. Check the attachment ledge for the upper bunk if you plan to use it. No mention of a
battery or any 12V system.
Tires (verify dates) are 5 years old- should have a year or two left, but I'd probably figure on replacing the two on the road if I were planning a long trip. Sounds like an older
paint job is just starting to fail- might have to deal with that at some point, but it doesn't affect your ability to use the trailer now.