I’ve been involved in an online shop in Thailand which makes and sells medieval clothing online. So my comments are based on that perspective.
We use a measurements instruction chart as well as a normal size chart (XS, S, M etc). In hundreds of sales we’ve only ever had one customer complain about sizing (we re-made the item and shipped it again, and that resolved the matter). Basically, if you follow the measurement instructions that the site provides, it should work.
Having said that, some clothes are better suited to online ordering. There’s a lot of leeway in medieval clothing, much less than there is in a contemporary men’s suit.
Like most online sellers, we have a very clear and generous refund policy, necessary to make sales online, it’s that simple. It’s not necessarily of our own volition: some jurisdictions (eg, EU) and most selling platforms require it. It’s also good business: if the client isn’t happy, their bad online review can be detrimental for the business, so a refund is always possible. In our case, if the buyer isn’t happy with the size or the quality of the item, they have the right to return the item.
If the seller offers payment by Paypal, you are normally (not always) covered by Paypal Buyer Protection. From the Seller’s point of view this is a pretty blunt instrument although it’s only happened once to us (United States Parcel Service lost the parcel). Paypal simply froze the money in our seller’s account until the matter was resolved. Buyers Protection is a good deal for buyers, but before you buy you need to check that you are covered by it. While I’m talking about Paypal, also make sure that your address in Paypal is up to date. Sellers ship to the Paypal address so if you’ve moved house and your Paypal address is out of date, delivery your order is unlikely to happen, and I doubt Buyers Protection will cover you.
I would also think twice about ordering through eBay. As a seller, we haven’t used eBay because they are quite rapacious compared to other platforms. As a buyer in the past, I’ve noticed that when something went wrong with my order, eBay’s processes were much more about protecting the seller than they were me, the buyer.
eBay give very limited space for reviews (80 characters from memory) and sellers on eBay who get a bad review also seem adept at re-listing the item under a slightly different name, and any bad reviews from sales of that item under its previous listing conveniently disappear. As a buyer, I now avoid eBay, and although I might look there I also look to see if a particular seller has another channel. If they do, I’d consider buying through that.
Social media selling sites such as Etsy can give you a better idea of how a seller is. If sellers on Etsy aren’t getting a majority of 5* ratings, I’d probably avoid them.