It is not a big deal, certainly not a reason not to buy the trailer, but it does mean some maintenance is needed, and preferably before the winter if you will be storing it outside.
If the plastic cup washer underneath the rivet head is still intact, all you need to do is pop off the cap and pop on a new one. You can order a whole set from Scamp
If the cup washer is cracked or missing, you will need to replace the rivet, adding a new cup washer and cap as you do. There are lots of old threads on how to replace rivets. It's fairly simple once you learn how, and it's considered normal on a Scamp to have to replace a rivet every now and then. All the parts are available from Scamp and you can get a rivet gun from a local hardware store. Everything is also available from online sources.
Once the cap is gone and the cup washer breaks apart, the rivet can become loose. That may allow water to seep in. It can also allow excessive movement of the riveted parts, causing the hole to enlarge and complicating future repairs. It won't happen instantly, but it's better to deal with it sooner than later.