Hello and thank you for coming to our website. We appreciate the opportunity to help you with your questions. Mercury is extremely toxic, particularly if you don't know whether it is mixed with other materials. Mercury formed into compounds containing organic components (called "organomercury" compounds) can be fatal in tablespoon quantities. Mercury can become airborne, so make sure that you don't open the container without wearing a surgical or paint mask to avoid breathing any mercury vapors. Many people have old materials stored in their homes that weren't controlled at the time they purchased it and toxins are frequently found. I'm not sure whether in Texas you can legally sell the mercury you have without getting permits and involving the Environmental Protection Agency and/or whatever Texas department handles toxic materials. Most people who discover toxic material in their home are only concerned with safely disposing of it. I haven't heard of anyone trying to sell it before. Your best source to find out whether you can sell the mercury you have and the regulations involved in that sale is to contact metal recycling companies in your area and see if they will buy it or at least dispose of it for you. If you try to sell it overseas, you are going to be spending more money on getting government permits to ship a known toxin overseas and paying a delivery company to ship a hazardous material than you will be able to earn from selling it. I would have to check the import/export control list to determine whether mercury is allowed to be shipped out of the US. Also, since you found it, you can't provide any guarantees as to its purity or any other materials it may have been mixed with, which will make it harder to sell.
If you can sell it, a metal recycling company will give you the most money for the least investment of time and money. Given its toxicity, metal recycling companies may not be willing to deal with it at all. Even if you can get a metal recycling company to dispose of it for free, you are probably making a good deal. If the local metal recycling companies won't even take it for free, then you will have to deal with your local waste disposal facility and find out what it will take for them to dispose of it. Make sure that you don't let any of it out of the container, because if any mercury gets onto your property, you will have to either pay a hazardous material handling company to perform an official cleanup of the area, declare the mercury spill when you sell the house, or take the liability of not declaring a known hazard when you sell the house.
Most people that find material like this when they purchase a house are more concerned with getting rid of the liability as quickly and cheaply as possible than they are about making any potential profit from it.
Contact the local metal recycling companies and they should be able to tell you about any local laws relevant to your situation, as well as knowing whether you can sell the mercury for profit.
I hope this helps. If you need any more information about this, please let me know. Make sure that you don't touch the mercury with your bare skin or breath any vapors.