The most expensive mistake at auction is not a cracked frame. It is a perfectly good bike in the wrong size, because that one is not refundable, not repairable, and not obvious from the listing.
Sellers quote seat tube length because it is the number printed on the frame. It is also the least useful figure for deciding whether a bike will fit you. Two frames sharing a seat tube measurement can place your hands inches apart, because the top tube length and head tube height differ.
The two numbers that describe fit are stack, how tall the front end sits, and reach, how far forward it sits. Every manufacturer publishes both on a geometry chart for each model year. If you know the stack and reach of a bike that fits you now, you can compare any listing against it directly, and the brand on the down tube stops mattering.
Sizing is not standardised. One maker's 56 is another's 54, and the same maker's 56 can move between model years when a frame is redesigned. Some brands size in centimetres, some in S/M/L, and some in both inconsistently. A listing that says "56cm, fits 5ft 10 to 6ft" is repeating a rule of thumb, not measuring your body.
Find the geometry chart for the exact model and year in the listing. If the seller cannot confirm the year, that is itself worth knowing, because it means you cannot confirm the geometry either.
Adjustable: saddle height, saddle fore and aft, stem length, stem angle, spacer stack under the stem, bar width and bar reach. Between them these move your riding position a meaningful amount for relatively little money, and a stem swap is the cheapest fit change available.
Not adjustable: stack and reach. They are the frame. A frame slightly too small can usually be stretched to fit with a longer stem and a bit more seat post. A frame too large generally cannot be rescued, because there is no way to shorten a top tube, and a stem short enough to compensate makes the steering unpleasant.
The practical rule when you are between sizes and cannot test ride: err small. Small is adjustable. Large is a resale.
Two tape-measure photos settle most of it: seat tube from the centre of the bottom bracket to the top of the tube, and the horizontal distance from the centre of the head tube to the centre of the seat post. Those let you place the bike on a geometry chart even when the model year is uncertain.
Ask what stem length is currently fitted, too. A bike wearing an unusually long or short stem is often a bike the previous owner was also trying to make fit.
How eBay bike auctions work covers the bidding side, andspotting carbon damage in listing photos covers condition. Once size and condition are settled, theShimano 105 board is where most sensibly-priced complete bikes turn up.