This page was inspired from Joel's Interactive Guide to Straight Razor Shaving.
The question on what to keep an eye for when buying a second hand straight razor is a regular topic. What a straight razor buyer should be aware of when buying that second hand razor on eBay or in an Antique Shop.  
What to look for
There are various bits that can go wrong when buying a straight razor. When buying a straight razor, make sure that you can have a very good look at it and/or get as much pictures as possible. If the pictures are blurry, out of focus or only showing one side of the blade, the sale should not be considered.
Also, be prepared that the final price does not include a shave ready straight razor in most occasions. The scales might need to be repaired or changed. Those must be considered before buying. The more damage, the bigger the restorations, the more expensive it will be.
The Blade and The Edge
The edge is of course the most important part of the straight razor. The edge cannot be replaced, any problems can only be honed out, reducing the width of the straight razor.
Carefully look at the edge for chips and cracks. Ask for more pictures if you are unsure. Keep in mind, chips will need to be honed out to make a shave ready edge. Cracks leave you at risk of a blade that might break while honing or shaving.
Those can be difficult to spot sometimes, but a close attention is always required near the edge and on both sides.
The edge is one of the most important part of the straight razor as you use it to shave. Make sure the edge does not have any pitting close to it. A razor can be saved if it has pitting most of the time. The Honemeister will need to hone the pitting out to avoid having any chips on the cutting edge.
Also, too much pitting on the blade might indicate a razor stored in poor conditions. The metal might be weaken and might not hold an edge.
A frown is defined by having a smaller width in the middle of the blade between the edge and the spine(Not smiling). To correct a frown, the Honemeister will need to make the edge flat again and then hone it. Meaning your razor width will be reduced to the smallest width. Most of the time, it can be repaired. If the straight razor is a smiling blade (Smiling up), there are no issues with that type of blade.
Hone wear is defined by how much metal was removed by the previous honers to make the edge shave ready. If the edge has too much hone wear, it might be close to the end...
The scales are easily replaceable in case of a problem. If they are cracked or broken, they could be swapped with another razor that have a similar size blade. This doesn't always work. A new set of scales could always be bought, but there is always a possible incompatibility with the blade. Having a set of scales made is a sure replacement but a more expensive option too.
Holes in the scales
If the scales have holes in them, bugs probably had a feast and they might be weaken.
Cracks on the scales
Depending if you are ready to have new scales made for the blade or not, have a look if there are any cracks or missing pieces from the scales. Cracked scales should not stop you from buying the razor as long as you are ready to live with them or replace them. Keep in mind that a cracked pivot will probably result in a loose blade.
External linksThis thread should be consulted in reference: