Its murky, theyre actively prohibited here, though its ambiguous if theyre saying no in a "at all" or "for profit" capacity.
You are right that valuation is going to be really tricky, especially as flavor of the month builds and patches ebb and flow. The focus is really defined by how much clutter needs to be cut through to get a clear picture. See, due to the values sliding on logarithmic scale the closer you get to perfect, with the valuation of off stats varying very widely from player to player (from no value added to exponentially perfect) its just not something that can be prechewed for the user.
Range filters and multiple layers of sorting would be the way I approach it- say Im selling a helm with near perfect strength, so I choose the slot and set the upper end of the str filter to perfect and the lower end to a bit below what I have. I sort by price and maybe notice that all the ones with crit are selling for crazy amounts of money, so I set the filter to how much crit I have (zero) and I also tighten the range to a point shy of perfect str since I notice the perfs are exponentially more valuable than what I have. I also might want to tighten up the date to the last few weeks, since I cant jump in my time machine and go back 6 months when anything that dropped from nightmare was crazy valuable(at the same time if I have something so crazy rare that it only drops once a year, I would want to see records of similar sales as well, no matter how far back it happened). From there I might have burnt away enough clutter to notice that people seem to be paying a crazy amount if it has a decent amount of stam+goldfind on it and I just happen to have a decent amount of both of those etc.
You might be able to get away with putting alot of these values into a tiny int, but many might need a small int for some values, or if stat inflation rears its ugly head. small money should be able to handle prices over money etc. Looots of indexes, multifield indexes, a table to track what people query for (maybe you want a way to have them send this to you? its the creme of the crop, the info to gear indexes to the program run most efficiently in future builds and info to corner the market).