I wouldn't say I'm a huge vintage shopper, but I get a good portion of my clothing from vintage shops. I mainly shop on Etsy for vintage because there are some amazing deals on there and I don't have a good vintage shop anywhere near me. Another point in Etsy's favor is that the selection has (for the most part) already been curated by the shop owners, so I don't have to spend my time picking through endless racks of clothing that may or may not be actual vintage.
Here's some tips and tricks that I've picked up through the years:
1. Know your measurements: This is so vital when shopping for vintage online. Vintage sizing runs differently than today's, so don't ever go by the size on the tag, go by the measurements of the garment. If they're not listed with the garment, don't be afraid to message the seller and ask, almost all of them will be more than happy to oblige. Measure your bust, waist, and hips and compare them to the garment to see if it'll fit. If you have wider shoulders, like me, you may want to measure your shoulder width as well to compare.
2. Condition is key: I try to only buy items that are in a "like new" condition or in a really great, hardly-been-worn condition. Again, don't be afraid to ask the seller a million questions about the condition or for more pictures. I try to avoid items with moth holes, stains, ink marks, missing beads/sequins, or other issues that can't be fixed. Is this always avoidable? No, I have been known to fall in love with a sweater or coat with a few teeny moth holes and end up purchasing. Use your judgement.
3. The tailor is your friend: Tailoring is definitely helpful when you're shopping vintage. Find a dress you love, but it's way bigger than you? A tailor can help reshape it so it'll fit you perfectly. I get a ton of my clothing (vintage and non) altered so it fits me really well. It may be a little bit more expensive, but the results are so worth it.
4. Narrow down the search: Use any criteria possible to help narrow down your search online. I tend to have a few fallback designers and decades that I use to help narrow down the selection, but you can really search by anything. If you're looking for a coat, try searching 'wool coat' or 'blue coat' or 'dress coat' or 'Ralph Lauren coat' rather than just 'coat'. This is really helpful if you're searching for something specific, but is also helpful when you're just browsing.
5. Buy what you love: On Etsy, you can add items and shops to your favorites. This is helpful if you're on the fence about something, just add it to your favorites and think about it overnight. The one downside is that it could be gone. I try to buy things that I either just love, can't stop thinking about, or are easily worked into a wardrobe of contemporary clothing. I tend to ask myself "would you buy this if you saw it in a modern store today?" and if the answer is yes, it's a winner. If the answer is maybe or no, then I think about it and most often pass. This helps you avoid looking like you're wearing a costume.
6. Don't overlook potential: Love a dress but really hate the long sleeves and think it would look better and be worn more with 3/4 sleeves? That's what your tailor is for. A good tailor will be able to do almost anything to a garment to make it more wearable.
I tend to avoid vintage shoes because worn shoes gross me out, but that doesn't mean you have to. I mainly shop for clothing, but really the possibilities out there are endless. Do a little research and know what you're comfortable with, then go shop! I am definitely not a vintage expert, but I know what I like, what I'll wear, and what looks good on me. I read all of the available information and ask questions if I need more, which helps me avoid problems most of the time.
Happy shopping!