Ricordea yuma or florida?
florida are pretty hardy and I haven't seen too many that look unhealthy being sold.
yuma on the other hand are notorious for being difficult to keep. i would agree that the mouth being open is a sure sign that it could be unhealthy, but not definite. even healthy yumas will open their mouths sometimes and so a healthy one could randomly have its mouth open for a photo-op. another thing to look for is damage to the polyp (usually from rolling around on a sharp rock during stressful shipping) you can notice these damages edges if the "little fingers" all over the yuma are not inflated in certain points. these damaged edges can heal but if there is a lot of damage to a yuma in a picture online and it has to be shipped, another daylong trip in a bag could be enough to kill it. the last thing i would look for is dark blotches. these are sites of bacterial infections. this can be overcome with adequate flow to "wash" the bacteria away. again, avoid buying stressed yumas that need to be shipped again before healing. whenever i buy a yuma now i ask the seller. some sellers don't even bother answering your questions and some are great and will not send moderately stressed yumas. i guess all yumas and vendors are unique. good luck and buy aquacultured in person to avoid all these problems.