This is definitely a fun niche in the reselling world. I have found my share of great beach towels and have sold them for a nice profit.
Look for BIG ritzy type names, vintage cartoon characters, and retro advertising…they can sell between $25.00-$350.00!
It would definitely be worth your while to look up “beach towel” in the “used” category on Ebay under sold listings. Lots to look for in your adventures!
Good luck finding one!