After trading with customised villager, item(s) that the villager sells is(are) successfully bought, however, the game crashes in a few seconds. Normal villagers trading does not have any problem. The command for summoning the customised villager is
/summon Villager ~-1 ~ ~ {Profession:0,Career:100000,CareerLevel:100000,Offers:{Recipes:[{rewardExp:0,maxUses:100000000,uses:0,buy:{id:"minecraft:leather",Count:64},sell:{id:"minecraft:leather_boots",Count:1,tag:{display:{Name:"Leather Booster",Lore:["Leather booster provides","tiny speed!"]}}}}]},Invulnerable:1,Attributes:[{Name:"generic.movementSpeed",Base:0}],Silent:1,CustomName:"Leather Booster Trader",CustomNameVisible:0}Also another problem is that the trade rewards exp even I set the rewardExp to false. Crash report is in the attachment, as well as a game screenshot.
The crash is cause by the value of Career (see attached crash report, Career was set to 5).
Career tag define the Career of the villager depending on his Profession (see wiki). In your case the villager Profession is 0, so his career must be between 1 and 4. Then rewardExp is one of the few tags that require type identification, so you must write "rewardExp:0b".