Regarding what #1 said about "you should definitely look at big department stores"... unfortunately i found them super disappointing last year. Inventory is always a mess and things sell out instantly. For a $500 budget, id look at these instead:

• Diane von Furstenberg New Julian Two Silk Wrap Dress: Way better discounts at outlets, though final sale is risky.
• Ganni Seersucker Check Midi Dress: Very cool vibes, but holiday shipping times are honestly the worst.

Building on the earlier suggestion, check out the Outnet. I usually grab a Theory Etiennette B Blazer Wool Blend for under 300 dollars there during clearance, which is way below your limit.

you should definitely look at big department stores like Nordstrom or Saks because they are so much more reliable for beginners. i was in the same boat last year for my cousins wedding and was really nervous about spending that much money online. i found that sticking to the big names is the safest way to go if youre worried about quality. i ended up getting a Nordstrom Signature Silk V-Neck Midi Dress Navy Blue during their sale and i was so satisfied with how it turned out. it felt way more expensive than it was and the return policy made me feel safe since i didnt know my size in high-end brands. for your 500 dollar budget those black friday deals are perfect. i also managed to snag a Tory Burch Robinson Convertible Shoulder Bag Black Leather for a great price that really pulled the whole look together. just stay away from random websites that look too good to be true... sticking to the big names works well and keeps things stress-free.